Two comment and a question:
First, you guys are awesome. I joined to forum so I could say that. I have at least one L that need re-powering, so I'm following your work closely.
Second, I recently installed at Lifan engine from HD on an old mechanical log splitter and must say that the quality of the engine is superb. I needed an integrated 6-1 gear reducer, which Lifan offers, largely for cement mixers, I presume.
Question: I have no engineering background or aptitude, so I'm asking a village-idiot question. Has anyone considered connecting an third-party engine to a Gravely L using a flexible coupling (like the one they once used on Craftsman table saws in lieu of a pulley-and-belt system or the one that was in the Erector set I had as a kid)? I'm thinking it would be easier to find compatible terminations for either end of the coupling than to do all the precise machine-shop work you guys are doing. Plus, if things aren't quite centered or lined up, properly, maybe a flexible coupling would be forgiving. There may be a thousand reasons why it wouldn't work, but if it will, how do you want me to make out the check?