The process of re-tapping a BSA thread in an already threaded metric crank isn't too daunting. All you need is a reasonable amount of skill, a good pair of taps, some cutting compound and some sort of jig or pillar drill to keep the tap vertical in all planes in relation to the crank.
When I started importing from France in the mid-70s nearly all the cranks, including the Suntour, Sugino and slightly later the MAVIC were French threaded..and MAVIC and Suntour were in big demand in the UK.
The upshot of this activity is that I have sets of specially machined taps with "lead-ins" precision ground on to them. So, Amir..if you cant get yourself fixed up with a good set over in your neck of the woods, I'd happily lend you some of mine..but you'll need to find your own cutting compound.
Norris Lockley..."Taps-R-Us"...Settle UK