645 M transmission bogs down engine

I want to preface my post by saying that I am a pretty experienced mechanic but I am not familiar with these machines at all.
So I picked this old 645M loader for what I thought was a good price, especially when I was able to get it running with just a jump start and some fuel but that is where the good news stops. It was only about 5 miles from my shop so I just planned on driving it to the shop and going through everything before I put it to work. When I tried to drive it, it moved very slowly and the engine started bogging down and black smoke just poured out of the exhaust. I couldn't even get it out of the driveway. I started with the basics, air filter, fuel filter, make sure the turbo is working, all with no change. We ended up hooking it up to my poor half ton Chevy in 4x4 low and started pulling it home. The engine died after 100 yards but my brother who was pulling me didn't stop. That was the sketchyest 5 miles and 4 turns I have ever done and that is a very high bar. I discovered that it is actually possible to turn corners without the engine running. After we got it home, we changed the oil, fuel filter, oil filter transmission filter, put a new low pressure fuel pump on and tried it again with no change. Today I pulled the strainer from the transmission and fell sick when I pulled chunks of metal that look like gears and a 4 in long very thin spring out of the bottom of the transmission.
Ok, that brings me to now. Can anyone help me figure out my next steps? I'm ok with rebuilding it if I can get the parts and a service manual. My budget is pretty tight since I wasn't planning on having to do any major work since the loader "worked great, just had a bad hydraulic cylinder" when it was parked. Do I just have a very expensive and heavy yard ornament?