There's a friend of mine that knows how to build wheels actually, hope he will be able to find time to teach me this winter.
It's dead easy, I did mine from online guides. If you decide to go ahead let me know and I'll post the main ones I used. Don't forget to budget for the spokes!
If you go to the effort of building I'd switch to alloy rims, especially if you want rim brakes. If you want to keep it coaster brake and have gears you'll need to get a hub gear with coaster. I have an SRC3 on my Kingpin.
Are those 20" BMX size (406 ERTO) or one of the vintage 20" sizes? If the latter it will be hard/expensive/impossible to find alloy rims - which of the three depends on exactly which 20" you have. 406 will be no bother at all.
Oh, and I love your bike :)