Somewhere in the pic at left, is a set of BSA A10 cylinders. In your pic, the studs should have stayed in the crank casings. The casings were not that great to begin with. I think on my BSA, I had the stud holes welded and re tapped. On a newer Norton engine I used helicoils.
The square plate on the bottom of the engine, can leak oil, if the studs work loose. It happens. Then you bugger up the crank bearings.
Have you considered a 400 Yamaha twin cam motor. The make about the same power, around 45, or a newer Kawasaki 500 twin around 60hp.