5 years ago, I had the same idea, luckily my buddy talked me out of it. He said “why try and re-invent the wheel, get a CR80” I got a CR80 and love it, parts are readily available, plenty of after market karting support, like motor mounts, pipes, shifting J-arm and counter shaft sprockets. A stock CR80 makes about 24 HP, I can keep up with most 125cc tag karts, but CR125’s kick my butt down the straights.
Good luck,
Gary
Honda CR80 #55