It's unfair for me to weigh in with my $0.02 without know all the details, but (t ... yes everyone has one and an opinion) I've been tending to my 77-vintage MD11 for 25 years.
If I recall, the transmission holds less than a quart of 30 weight oil, and its innards consists of brick s-house-sized rotating parts. Unless there are broken gear teeth, I'd just flush the transmission with diesel fuel, and refill it with oil (maybe add some STP oil treatment) and if it doesn't sound like it is grinding gravel, it might run another 20 years !
Volvo parts cost too much. Look for a junker engine at scrap metal price. If you look hard, they can be found. Most MDs are trashed because of raw water cooling issues; most of their rotating parts are far from worn out.
I too, had a recent issue of "it stopped and after a while, would start again." After months of changing everything, I discovered that wasps (or some other insects) clogged the vent lines on both fuel tanks. The clogs weren't perfect, so after a while the air would seep past the clogs and the engine would start.
I have two fuel tanks, two filters, two of everything feeding the lift pump on the MD11. With all lines open, I figured that if one fuel system (side) failed the other side would take over. And, I was right. When one side failed (clogged vent line) the engine kept on running using the other side..... until that side failed. Since I don't burn a lot of fuel, I had no way to know that the first side failed when it did.
Ed
Caprica #687