noobie trying to save a 92 k2500 6.5 TD

New guy here. I have come into at a very cheap price a 1992 Chevy K2500 4wd 6.5 turbo. The previous owner ran filtered waste oil (15/40 drained from otr trucks) with autogas mixed to thin the oil. Last week it wouldn't start and ended up breaking the starter bolts and trashing the flexplate ring gear. The truck ran remarkably good on this fuel up till it quit.  What could possibly be the cause of not running any longer?  How difficult and what do I need to do to remove the auto trans to replace the flexplate? (He gave me a new one). So far I have removed and cleaned the fuel tank, added fresh fuel, (diesel) and pumped (by connecting lift pump to 12v) the fuel to the fuel filter and filling, and a new filter, and fixed starter (added the bracket also)  Before I try starting it I need to replace the flexplate. Should I also remove injector lines and bleed the whole thing out? I hope the injector pump is still good. The pump and injectors are from a marine 6.5 engine. Not sure what advantage that is. Can any body offer suggestions?  

check the valves to make sure nothings damaged from what happened  
I burn more diesel and run more miles in a week the most of you people do in a month. ctds that have been used 97 3500, 98.5 3500, 98.5 3500,01 3500, 03 3500. those that are being used 06 3500, 07 3500. and yea these trucks earn their living

First, replace the flexplate.  It shouldn't be too hard if you have a transmission jack.  Hopefully you have the room to just move the tranny down and back some to get at the flexplate bolts.

Once the flexplate is replaced, I'd pull the injectors and bleed the system.  And while the injectors are out, it would be a very good idea to do a compression test.  You'll know if you have the proper balanced compression (and thus good valvetrain).  If you have one cylinder with very low compression the previous owner might have managed to bend a rod.

I'd suspect a leaking injector which let fluid in the cylinder when the truck was sitting and not running (possibly from the use of a "custom" fuel blend).  The truck tried to compress the fuel...and thus a no-start condition resulting in a broken flexplate.  The flexplate and starter damage means the starter was doing everything it could to rotate the engine.  But something made the engine extremely difficult to rotate thus the starter/ring gear gave way.  I'd say you'll be lucky if you don't have a bent rod (though with a diesel they are much sturdier than a gas motor) and only had to fix the starter/ring gear.