There are some things to look at prior to taking on this task.
Eric Gorr talks about frame breakage on the 94 frames at the point where the rear shock mounts at the top. He suggests adding some gusset plates to fix it. Also the upper chain roller was flawed and the chain hit against the air boot. The solution is to turn down the chain rollers's diameter on a lathe so it can free wheel. Even with that when you put your kdx engine in you should make sure the chain won't contact with the kdx boot. I know Fredette has put kdx's in late-model KX's and someone has probably done it with the earlier models. Have you even taken the engine out and laid it in the 94 frame to see if the exhaust will fit nicely.
Also, one of the magazines characterized the 94 kx 250 as really liking the bike. It had great suspension and won the 250 shootout that year. Just to give you an idea of what great suspension means, for the 93 bike they said that the front forks would stand up against all modern (98 and below) dual-chamber super forks.