Hi Kristian,
wish my Dad had done the same! The good news is that BSA parts for bikes such as the A10 are readily available from a whole number of places. You won't have any problems there. As for the fuel, there are many opinions, ranging from running it on unleaded, running it on Lead Replacement Petrol (LRP), adding an additive (my favourite), or having the valve seats replaced.
Oh, and by the way, if it's an A10 it's a 650cc. The 500cc version is an A7.
Good luck
Nigel.