I am wondering about this too. Which shaft size to get and what will work with the ace redrive?
I recommend watching Les Homan's video series on the BSV23. In short, the Ace Redrive is agnostic to the size of shaft, so you can do a 1", 1-1/8", 1-1/4", etc... and shaft length around 3"... If you can find a good deal on an engine with a 4", you can always cut it down to the length you need. You will however have to purchase the drive sheave specific to your engine and they are keyless and use internal pressure to hold onto the shaft. Here is an excerpt from the BSV23 plans that are available at https://legaleagleairplane.com/product/briggs-and-stratton-engine-plan/ :"Climax Metal Products C405E-112, for 1-1/8” output shaft, the number C405E-100 is used for 1” output shaft."Recommend buying the plans if you haven't done so already.