This is a common problem with a model with a long nose, operating on grass, that has it's wheels too far back when measured from the wing L.E.
Solution?
Fit bigger wheels or bend the legs so that the centreline of the wheels are further forward. Either way it's going to give you a headache as far as the proper retraction of the gear goes and make fitting fixed gear look the simpler solution.
As for putting on fixed u/c, well, the original Chipmunk had fixed gear anyway.