You would definitely need to find a frame and fork with compatible steerer tube diameters and lengths. I'm NOT a BMX expert, but I think they, at one time, had smaller steerer tubes than typical. If that is the case, it may be difficult to find a 26" fork. I was working with a 1987 Ross mountain bike earlier this year, and the stem was something like 3/4" (not the typical 7/8"). Someone mentioned that it was the size of a BMX stem, and that's all I know about that. Even if that once was the case, maybe newer BMX frames and stems are conventionally-sized.

Good luck. Sounds interesting.