Cylinder/head & weight of some 100 cc cubengines can be fitted in 70CC block with minute changes so may be the 5 speed gear if exists in 100CC engine wud be able to fit too.
First find such engine & than phisicaly inspect from outer side if its seems identical if yes than next task is to be done by some expert mechanic but still i dont think it wud be easy or bolt on type job.