Hmmm. Re-stamping really is the easiest option I think. At the end of the day it'll be as legal as anything else!
You can fit the MC28 swinger to the MC21, and I don't think it's too hard (though haven't tried myself).
As for fitting the digital clocks etc, Matt @ TYGA will definitely know more, but I'm sure you could transplant the wiring loom from a 28 to a 21, though how involved this would be I don't know.
Surely if you've built the bike from a box of bits, you haven't imported it, and can therefor claim it's a new build, not an import?
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball