Man Mauser hope that's not something common to that model as I have an RT3 and have never had anything like that. Curious that it is the second time for your bike...I'd be looking at something else not right that is causing the internal snafu. All the bikes can spit back a bit but shouldn't be breaking internals!

Maybe the kick drive gear is hanging up...weak spring of worn shaft?  Look closer at that kick gear hanging up. Above my pay grade but something else going on there.

Check the parts view and be sure you have all the pieces like shims and such too.

Wow, something I'd expect from maybe a Chinese import but not from a trusty Yam 

That’s a shame @ been there with my 490 that arrived from the US with a broken crank key so same result damn it. First thing check crank key before starting after new parts fitted. Secondly carefully check the gear behind the basket plus the kick gear itself. Good plan is to run the three gears around a known good gear to feel for a bent tooth & doesn’t need to be very bent to feel it. Trouble ‘is’ at this point you really need the new idler gear so it’s a ‘known good one’ to then use that to run the rest around. My 490 kick gear although looking ok, had a bent tooth. Anyway, feel ya pain. Keep the timing back, like 12 thou not 15 to limit kick back. Could mark the flywheel & strobe it to see it it brings it back to less than 2.9mm btdc. My old RT1 had a retarding centrifugal cam In the flywheel which they shoulda kept on RT2 & RT3 as woulda saved a lot of grief. All the best from NZ down under—& check that flywheel key!!.

Better revolve the shaft too to see if it’s bent for the idler gear. Also the bearing behind it which can split the outer ring but think they strengthened the case to support it better so is unlikely. That might’ve been a very first model problem. Bundle of joy aren’t I lol.

In the 1950's, 60's, and into the 70's many Japanese imports had the same "poor quality" reputation as Chinese imports do today...

Mucking around on my phone & never saw the other replies but i’ll find a pic of how tight I do the flywheel nut. You’ll be impressed (not). Worked on these at the dealer from the very first model so have seen a few damaged cranks from home repairs.

Ok ya can laugh lol, but if your crank is damaged & you can only get it looking reasonably good, Tightened very tight will still hold. I used to run my TT250 with no key while experimenting with more advance & never slipped & big heavy flywheel on those. This pic was my RT1 & flywheel was the advance retarding auto points cam. Can see the different shape of the face of the flywheel. Could just about start it by hand with decomp on--mind you it didn't have much comp but went good. 

Hmm, maybe just unlucky after a few kickbacks over time. Still think if you’re not keen to pull the flywheel at least check the timing as best you can. I dunno, easy way I have used is use a nail & get as straight in angled plug hole as you can & mark tic just rocking it to the high point then go down 3mm (2.9) & mark the nail. So points should be breaking at that point but that’s absolute max for kickback. If they’re breaking back before 2.9 then check the open points gap & if wide past 15thou then bring them back. Just really trying to get a handle on if the flywheel key is ok re kickback. Which I guess it should be if you’ve had the bike a while with no issues. Re decompressor, it’ll still kick back but will soften the blow. You’ll know if it’s working (not blocked) just by feel on the kickstart if you kick it softly not as to start & feel the difference. You’ll notice it running too by sound at idle & low revs as sound flat & fluffy. After all that, I don’t have a decompressor on mine. just kick it hard & it starts.

I obviously don't know your process, but kick start technique has a lot to do with how any large bore kick start's success rate is. I restored a '72 RT2, and did not try to locate to replace the entirely missing original decompression setup. Even with timing set to where it needs to be, a "lazy" kick would result in a reversed kick start lever. But a determined, full sharp stroke kick from the tippy top through to the bottom every time never resulted in kick back. I taught this technique to my buddy who I sold the RT2 to, who had never, ever ridden a motorcycle after 50 years of life, and the only time the kicker would consider kicking back on him, was when he got lazy. He learned...boots always required, tippy top the kicker, and a determined kick through the full stroke, and here we are 4 years later and he's a MASTER of the kick start. We ride alot too, he's put around 2000 miles on the RT2 and never an issue. Big bores require determined kicking to prevent broken parts and ankles.

Hi Mauser.
I have read a post on here some place that shows a picture of a members ankle severly bruised . The bleed had gone into his shin and muscle it was so bad.I seem to remember Makos name on the post.
I would look at it this way.At least it's not damaged the cases by cracking them.That happens and i know that.
The Maico 450 i had has given me a permenent injury through PO not tightening the motoplat unit on the end of the crank.
It kicked back so hard it smashed my knee and tore the liggaments in my ankle . It sounded like a gun going off it miss fired that loud. Yes i did have boots on.
Yes the 360 damage cranks too. I have a crank here that will need spray welding to repare it . It's snapped the key and trenched  the crank.