I see no reason a gunsmith or a good hobby gunsmith could not change a mauser magazine for taking 20 rounds magazines - FAL or G3.
with cut and tig welding only the sky is the limit!
years ago when the repro mauser trench magazine was not available I made such a thing for the owner of a FR 8. I cut a 25 rounds Dreyse machine gun model 13 magazine under the position of the 5. cartridge and we let it tig welding under a mauser magazine so it had 25 rounds again after weld together.

similar to this here

of course, its fixed but I am 100% positiv it can be done for detachable magazine too