I have seen a 16 with 21 bodywork, but not 18
I did look at fitting 18 bodywork to a 16 once. It looked like the lowers were almost the same. The problem comes when fitting the top cowl. The 16 stay is a lot taller than the 18, and from what I can remember an 18 stay is not a simple bolt on job.
The tail should fit, may need the sub frame as well.
As for electric start, NOOOOOOOOO.