I use a late model CRF450r master cylinder on my 01 kx500.
My brake caliper is in excellent condition, I use a over sized rotor, and a SS brake hose.
I didn't notice much improvement with the CRF master cylinder but no issues either.
A recommendation is while you are replacing your master cylinder, disassemble your caliper and at a minimum clean your seals.
Few ever do this and it makes a big difference in your brake performance.
Before:
After: