Hi Dave..You can use a 'long conrod' pre 1954 B33 piston..A +.060" one is 86.5mm and will give you 658cc with the M21 stroke.
I have bored these barrels to 90mm quite a few times now and haven't encountered any problems. There is a fair bit of 'meat' in the casting and the cylinder is also supported and strengthened by the tappet chest casting.
I've been running my own 720cc (90.5) engine for about 8 years now, maybe more and up till quite recently had two bikes in this format. I have also built a number for other people. I ran a 47 M21 with a B33 piston for around 7 years as a daily hack..The only thing you will need to do is fit a solid copper head gasket and cut it back to the bore diameter to avoid any possibility of piston to gasket contact at TDC. As Neale says Ducati pistons are worth a look, the 916 has varying pin sizes for different years (19/20mm) but they will all go into a SV small end bush (but not into the OHV one). However, you will have to check the deck hieght. Some of the current batch of big singles also have large bores, but the pistons are usually very short and frequently only have two rings. With a short skirt in such a long stroke engine they can be noisy when running. With a modern forged 90mm piston, which has a much lower expansion rate than the original cast ones, I bore and finish hone them to .004" skirt clearance (.0035" in my own engines as I tend to run them in very carefully)..With a B33 piston you can go with std B33 clearances and you'll be OK (I make those recommendations on the basis you will follow normal running in procedures).....Ian
email (option): ian@wright52.plus.com