Yeah, all 60s/70s Jawas are pretty cool looking but slow. Perak (pillar suspension) motors are interchangeable with kyvacka (swingarm) motors with a few exceptions (different air intake/air cleaner and slightly different cases. So if a motor fits into a Perak frame, it should also work with a Kyvacka frame - the tank might be an issue, though, since they are different.
Those 2-stroke motors were designed in the 30s and only updated slowly until they stopped making them sometime in the 90s.
There was a Kyvacka that I would see around Prague with some kind of motorcross engine - I never got a good enough look at it to see what it was.
There was a 150cc version with slightly smaller frame that had a CZ motor in it - the 125 & 150cc versions were marketed as "Jawa CZ" bikes since the motors were CZ & the frames were Jawa. The 150cc motor actually felt faster than the 250 version.
I'm sure an RD motor would make a great conversion. The only area I would worry about interference would be the frame backbone - I don't know if you could squeeze the RDs carbs back there.