The standard 2A1 mag was 10 rounds but there was a 12 too. You'd need to ask whoever rules NYS if blocking a mag makes it legal. Lotta daft rules around. Up here, rifle mags are limited to 5 except for M1 Rifles and Lee-Enfields and a few others. All are exempt by name. A 20 rounder for an M-14 or the like is worth 10 years in jail. Mind you, a real semi'd M-14 is evil now. So is an M1Aicon.
I suspect asking a smithy to do it would cost more than the mag is worth. Might and probably is just a change of follower. Dunno for sure. And a mag in pieces is still in your possession.
Gunparts lists new GPC, 10 round, mags at $56.25.