Another to add to my collection.. 1974 DT250A (I know.. theres a few threads going about them)... 
I finally had time to remove the motor... I'll store the rolling carcass in the shed this winter whilst tinkering with the engine..
Sneezles61

Well, I bought it with the head removed.. no big deal....
 I got to looking to see whats in the boxes... Old dirty bike parts... most of them any ways... 
I noticed the chain/sproket cover was... well.. missing lets say... 
Then, I put the engine on a rolling cart just for MC engines.. I think I see a problem... I won't attempt any thing with the head until I find out how bad this is..
Sneezles61

Ouch!

Well, I got the gear selector shaft out... The bend in it, sits inside the lower casings... and it has been this way for a while.. There wasn't any oil in the tranny... perhaps it leaked through the shifter... So now the bore/hole that supports the left side is quite out of round... 
What options do I face?  Bore it out and make a brass sleeve to press fit? Was thinking making a snug stainless steel washer to keep it centered and snug enough to be kept in alignment via the aluminum recess that keeps the seal in place? 
Sneezles61

If it were me I'd open the bore from 14mm to 18mm.
I would counter bore 20mm diameter x 2mm deep.
Make a brass insert with top diameter 20mm x 2mm thick and a lower diameter 18mm with a length to suit the crank case and a through bore of 14mm to suit the selector shaft.
Doing it this way means you wont need to bond it in or worry about it pushing though and the amount of material removed shouldn't effect the case integrity.
I'd make the 18mm diameter a minimal interference fit but not the 20mm diameter as that's only really there so the bush has a stop.
That's what I would do, I'm sure there's other ways to rectify it.
 

I wish I would have thought to say something sooner.  May not have helped though. 

I've had luck with cutting off the shaft as close as possible to the case and then using a die grinder on the stub until it clears the case.  Tedious but it works.  (I have an aggressive bit and a high power grinder...  not a little dremel)

A friend heated the shaft and bent it back with a pipe and called it good... but his wasn't as bad as yours and he didn't disassemble...  he just replaced the seal and kept riding it. 

 

Make ya wonder what impact bent it like that. I guess being wasted in the middle area makes it easier to bend though.

I wonder if something quite heavy fell on it...  How could the sprocket cover be in many pieces, the shaft bent.. but not much else mangled around it? 
I didn't get the expansion chamber for the exhaust system.. the silencer is here tho... 
 Well, I have a gear sector shaft coming.. and a seal for it... So I have some time to ponder what the fix is for the case... 'llI see if a machine shop is willing to attempt a bushing.. 
Sneezles61

Well, I see Bill Bune is about 150 miles from me.. I've sent off a text to start my adventure in a repair..
Sneezles61

Now for a newby question.. The DT 250A I bought, came with the whole top end separate... I see the speedo sez 6300 miles.. I do not own a very accurate bore measuring tool... so, "just" to get my head wrapped around the next phase, Am I able to measure the ring gap to help me understand how worn the bore is?  I'm thinking to put the ring in the cylinder.. use the piston to keep it somewhat square with the world.. and push it to measure a few spots in the bore...  Maybe I'm wasting time?
 I could send the the whole thing off for work... but I would like to know whats really worn... Heck, maybe even a good honing would work to get it running for a few years before it really needs some boring.. 
Sneezles61