If you can't find the original ST Wrist pin, you may try to use an Enya part. The head for the Enya 45 6001BB and 45 6002 fits perfectly the ST G21 46. Enya still have all the parts, including pistons and wrist pins brand new for these two engines, and perhaps they will fit the ST G21 46 as they are very similar. Randy Smith now can order parts directly from Enya for you.
Good Luck,
Martin