RT1 and DT1 have different Bore Strokes.
The interchange of parts is pretty amazing.
My RT1 has MX250A gearbox, YZ250D clutch plates and friction plates with springs, IT250D Stator and Flywheel, YZ400 CDI.
The cases are RT3.
The RT1 frame and wheels have some differences to RT2-3 but nothing that is not allowed under the Regs.
If you have the time, you can chase down the many different gearbox ratios in the MX250. Eg you can make a super close ratio if you want.
If the time ever comes and I find myself not racing MX, I'll register the RT1 (I left the lighting Coil in). Make a hell of a street bike at the lights.
Good Luck with it.
Peter B