How about spreading the current forks to 100mm? (Many LBSes will do this if you just bring in the bare fork. If you bring the whole bike in, they might not want to do it because of warranty issues. I've spread forks before using a 2x4 and copper pipe. )

The only other issues would be whether you significantly alter its folded state - i.e. whether it would stay folded - and/or whether the spread forks still fit 20" wheels.