Well, the Mark 1 mags are 9 rounds. The Mark 2 and 3 mags are 10 round mags. I believe all have the same exterior dimensions. If it was me, I would just buy a new mag.
We're talking about a regular MkI, right? Not a 22/45? My MkI mag from 1955 is 10, but a little snug. The springs are not similar, but the mags are interchangeable. Old springs fill the mag, new ones are circular ones up front. You have to disassemble a II/III mag and put the little protruding nut on the other side for a MkI. Then I had a mag release issue that needed a little filing. The plastic floor plates kind of scootch forward a bit and the catch snags the edge of the body.