The piston will be the hard part with this project the dome is a little different so will have to reshape the dome in the head more then likely sticky sandpaper on the dome of the piston work it in the head by hand maybe.
But on the plus side it's windowed and thinner rings
By the add it indicates the piston is a non stocker. Where is the pin located in relation to the stock A/B pistons? I assume the jug is a stock sleeved unit? Do you know which one, gt5 maybe? All this will come into play setting it up. With the larger displacement and the reeds should be a real torque monster.
I have interest in this build as I am about to start a Frankenstein build using the 54mm Stihl piston/cylinder on a 40mm bottom end, and will have to make some of the same mods among others to make it work.
Guess we won't really know what it's going to take to accommodate the stroke until you start the build. Jug spacing I am assuming. Between the spacing and the reeds being so far out of the intake crankcase compression will suffer. Consider stuffing the cases. If the piston has clearance to extend out of the jug with out hitting the crank that will help, that's why it's relieved I guess.