Pistons go up in increments of .010" so the first rebore for you would be +.010" providing the bore is not worn too badly, if it is then you will have to go to +.020".
Now that is all very well but finding a pair of +.010" pistons could be a problem, plus 20's will be easier to get.
There are several brands you can go for, JP's from here in Aus, GPM's from Italy or Wassel (Hepolite from Tiawan). I have used GPM's and I think they are a well made piston and will use them again. I have not used JP's in a A10 but have in a single (B31) with no problems. I have also used two sets of the Tiawanese pistons and both bikes are running well, I like the piston but there are issues with the rings. Both sets I have used I replaced the rings with Hepolite ones. Having said that I believe that the ones Wassel are selling under the Hepolite brand are being supplied with American Hastings rings which should be ok.
http://www.ebay.com.au/itm/Hepolite-piston-kit-11062-20-To-fit-BSA-A10-650cc-/330909738730?pt=UK_Motorcycle_Parts_13&hash=item4d0bc1e6ea#ht_1966wt_966The first thing you need to do is get the bores checked to see if they require a rebore or not, can you feel a lip about 1/4" down from the top ? You will need to get a machine shop to have a look and measure the bores to know for sure, they could be worn to a oval shape.
If the bores check out ok you then you can put std pistons back in. For a 51' I would recommend flat top pistons which will give you a compression ratio of around 7.5-1, that ratio in conjunction with a 356 cam will give you a very nice engine.