So I bought a M21 crank case set with a better condition barrel that I had. The part lot also came with 1 and 3/4 "BSA Gearbox", turns out they are C11 boxes. Looks like I will clean them up and make my money back. Grabbed a few things off the other cases that were missing on mine, such as the exhaust tappet adjustment bolts and the studs for the magdyno.
I also had the liner pressed out of the barrel leaving me with a 88mm hole to be bored out and honed. This is an "ol' trick" I heard about from a few people and should bring the engine up to between 710 and 750cc depending on the size of the bore.
So yes. My mind has been made up, I will be building the flathead big bore engine with a bit of work to the head and a modern piston to raise the CR to a decent number. Stock is 5:1, I want to try for 7:1
Ohh and to chronicle my BSfield engine I will leave some info on what I have found to be the combo needed.
M20 lower end (due to 94mm stroke, allowing for a shorter engine.)
Enfield GT 87mm piston (Giving a close to square engine size)
This will create an engine with 560cc.
The reason I chose not to build this was because I do not want to have to cut and bend my frame to fit the oil pump into the frame. If I was to I would much rather trade the frame for a single frame, most of which are used as Goldie replicas.