Had my bike to a area mechanic. It now runs (yay!) But we have some deeper issues. He suspects the bottom end needs work like crank seals.  Says splitting the case is tricky and he doesn't work on anything that old.  Fair enough.  
what's my next step? How hard is replacing these seals and bearings?  

That's a blow. But if you've not been into anything like that & your area mechanic won't tackle it, i'm not sure what to suggest. There is special tools needed & if the bigend is in need of replacement then extra special tools required, hydraulic press etc. I've probably done 50 in 50 years & the first one is always the most difficult. I've just google my eyes out trying to find a video of a 250 or 360 motor dismantle including crankshaft but nothing close as you need the right model for reference which is either 250 or 360 from "70 to "73. I've not mentioned "68-"69" due to a slight difference in the selector & drum for how 4th & 5th gear are changed. Is still mostly relevant though for the important bits. Goodluck with whatever you decide.

Seals can be replaced without splitting cases.  Bearings cannot.  Why do you think bearings are bad?

It was a guess based on age as far as I know

Rust is what normally kills the crank bearings as most don't have nearly enough miles to wear out the tough ball and needle roller bearings used.

You could pull off the top end and check the rod bearing pretty easy...  and check the flywheel side of the crank for play even easier. 

Question is if you want to try the work yourself or you'll need to find a new mechanic?  These bikes are not that hard to work on and learn to work on if you want to do it yourself.  Tools can be bought to do the basic disassembly and assembly for a couple hundred or so...  pressing apart big end of crank if it needs rebuilding should be left to a specialist...  most of us ship crank to a shop for that. 

Only you know your capabilities and desire to repair yourself.  Plenty of help available here if you decide to tackle it.  Or post up your location and maybe someone is near you that would be willing to help out?

If it doesn't sound like a concrete mixer it could get better with use. Video with good audio always helps if possible.

i got her home and it did start.  Reluctantly.  It runs a bit with lots of throttle but dies quick.  I'll see if I cant take a video this weekend.

I'm in far NE North Dakota.  

Far as I can tell, lots of neat little shops across the line in Canada.  Can't get there with the doggone border closed...

 

Beware the fat guy on the green bike

Still trying to figure why the video is back to front haha, how'd you manage that. All good & keeps us guessing. First thought--left hand kick start--what the. Anyway--getting sidetracked, is it kicking back or did i just imagine that each time it stops. If so, pull the flywheel & check the crank key. Otherwise get into the carb. Are you choking it as should start & run with no throttle but if it then richens up after a minute then close the choke. Sorry but a bit hard to exactly tell whats going on as doesn't keep going for long enough but "think" it just leans of out of fuel but not 100% on that.

Hi,
i had the same issue on my ´75 DT250.
Hard to start and running only a few secounds.
In my case it was the ignition coil. The spark was there but too weak. For testing, i tried the coil from my Suzuki RV90 and the spark was strong and bike run well.
But finally bought the VAPE 12v system to get rid of all the old stuff.
Regards from Germany