HT is the acronym for "Happy Times" engines, which is the label applied here to the really cheap (and cheaply made) 2 cycle chinese built engines which are probably the commonest after-market add-on power choice for bicycles.
A "rack mount" is a small engine mounted to a modified bicycle rack, either front or rear, and usually powering the bike by applying a driven roller against the tire to cause it to turn (friction drive). There are also rack mounted chain drive bikes, both regular chain (right) side, and off-side diven sprockets. The choices are various. Look around the site, read up on various people's implementation of the many choices, and decide what is best for your needs.