Most of the conversion kits I've seen are for higher end models to take golden dollars, but I can give them a call I guess.
I did figure out how the mech works (it's remarkably simple, checking coin diameter only) and I can get it to accept nickles with some scotch tape with the caveat that it will sometimes start accepting pennies if the tape gets loose. It doesn't look like it would be easily retrofittable to anything else, though I guess a custom plastic piece could be glued in place.
It may be cheaper to just buy a nickle mech, especially if I can actually find one used.