Now that I have become sucked in to this project, I thought I'd post what I would do. Such choices are always personal, and perhaps idiosyncratic, but I'll put this possible solution on record.
Step 1: Make a chalk mark on the existing engine's output shaft, and rotate the engine until the output shaft has made one full turn. Note how far you turned the crankshaft to do this. That will tell us the output gearbox ratio. It may or may not be 2:1 - we need to know for sure, to perform Step 3 below.
Step 2: Unbolt the current engine, move it backward 6", clamp it in place, and see how the mower feels with that slight rearward shift in center of gravity. Remember, it has to be reasonably stable, with weight on its front roller, when tilted backward 15 degrees, simulating climbing a maximum slope.
Step 3: If Step 1 indicates you can move the engine backward, that is the solution so you won't need an engine with a reduction gearbox. Source a double V belt pulley of the diameter of the current mower shaft one, one multiplied by the gearbox ratio determined in Step 1.
Step 4: Buy a new or second-hand engine (I'd choose second hand, of course) of about 4 or 4.5 hp, with an output shaft long enough to suit the V belt drive. Cut a piece of 1/4" steel plate that will cover the existing 4 engine mounting bolt holes in the mower frame, and provide room for 4 new engine mounting holes sufficiently rearward to allow the engine's output shaft V pulley to clear the large new V pulley on the mower drive shaft.
Step 5: Weld the plate to the two mower frame lateral rails. Bear in mind that one day, the welds will need to be cut to restore the machine to original condition. Drill the four mounting holes for the replacement engine procured in Step 4. Put it all together and go looking for a 15 degree up-slope so you can see if you got it right.
