and yes ray you and i need to get together and bounce some ideas back and forth. meanwhile keep an eye out for a cheap berk 6 for me
Not to plug, but get with Tom at JBP. His Aggressor endeavor early on showed pics in the super secret catalog of a twin 6J-sized jet setup.........dunno if he plans to make 'em or not tho, but would be a good shot at maybe sumpin new layin around or parts for such a beast.

I don't see any reason why a low end 50-60hp, or even well over a 100HP Snowmobile, jet ski, or motorcycle engine couldn't spin a 6 sized jet drive to the freakin moon and back. 2 cycle would probably be the way to go, but these newer 4 strokes in those things are amazing! I wouldn't rule out ripping a motor out of a sizeable Eggbeater either as a possibility (Pat leaves his unsecured quite a bit.......).
I would also consider using a driveline coupler from these newer Camaros and the '04-06 GTO vs a regular driveline. Kinda surprised noone has done it on a 12J drive yet, but that'd mount the engine crankshaft right on the jet input shaft, with the right custom adaptors. No U-Joints required, and shortening the engine compartment needs considerably.
Those little jet drives were built to run behind things like Kawasaki industrial engines (What the original Mule and JD utility ORUV's used), or Wisconsin twins ( Basically oversized twin and triple cylinder Briggs and Strattons) available at the time. But I also see no reason traditional thinking on impellers, etc we use on 12J equipment would be that much different, just scaled down.
Impellers, wear rings, etc, etc.......well, you really need to get with Tom on it I think. Obviously Bob has some experience with them as well, but he probably sold them to buy new toilets or bathroom tile or something...........

Ray