RE: HP VT21 worth using?
I've also got a VT21 I bought in the 80's and although it did some time on an airboat, it never saw any airtime before I put it away.
I got it out a few months ago and found that the bearings were rusted to hell and back despite using an after-run oil (should have used castor oil eh? :-) so I'm in the process of rebuilding it.
From memory they're not awfully powerful and (unfortunately) don't have the traditional 4-stroke attribute of liking to swing a big prop slowly. In fact, if I recall correctly, they take about the same size prop as a good 21 two-stroke but don't spin it nearly as fast.
They also don't idle particularly well.
But hey, in a nice high-wing cabin model (perhaps a (semi-)scale Piper Cub or Citabria they'd be a quite nice little mill.