Howdy. I've never done a project like this, but I wouldn't expect much if anything from a Ninja to bolt-on to or fit a KZ250. You might be able to use the meters, but you'd have to fabricate a meter bracket and do some minor rewiring for the meter lighting & warning lights. Depending upon the models, sometimes a caliper bracket for the brakes can be fabbed. But instead of trying to fit the brakes or swap forks, what's usually done is to swap front ends >> All Balls Racing has a fork conversion page. If the swap is possible, then you'd probably have to get the donor steering stem machined to fit the steering head on the frame. A front end swap is not uncommon >> complete fork assy, Fr wheel, Fr brake. often times referenced as part of a "street fighter" project.
ps: you could look into getting the Ninja cheap, then finding an engine for it. a modern motorcycle instead of an old clunker might be preferable.
