May be a while before I actually begin work, but preliminary measurements seem to show that the 20hp Perkins 3 cylinder diesel I have (bought from David, member "TheFarm 2004") will shoehorn into my Allis 620 without major frame modifications. I also think a Honda Civic radiator will fit inside the hood well. Won't truly have any definite answers until I have time to pull the Onan engine and lower the diesel down in there to check clearances and alignments.
I'm just now finding myself wanting to get into another project after a couple or 3 years of life's problems stifling my ambitions. It feels good just to start thinking of such a project, so hoping once I actually get started that I find myself having a good time again tinkering.
Only kicker to this project is that I sold the 3pt lift off it a few years ago, but at least I kept the rear PTO system completely intact instead of selling that off it as well. If I get this project going, then I'll have to build a new 3pt system for it.
Pics of the 720. She's backed into a shed & filthy at the moment.