Yves As the difference in bore is 14 MM it woould seem this would be too much for an overbore so I suspect the cylinders are different internally . As far as anything I have read all else internally is the same . Someone with more knowledge will be along soon I suspect and set me straight . George
Tollgate Classics here in England were offering a 450cc conversion for 350's. Which would suggest a bore of 79mm (ish).I don't think they stroked it. Perhaps a BSA B40 or B44 piston might fit (these being 79mm std)depending on the gudgeon pin diameter. Depends though, how much meat there is in the cylinder.