I built a Williamson Ringmaster many years ago. Williamson also designed another model -?Attacker?- also published in MAN in the OT era. Very similar, built-up fuselages, but different canopy placement and a diagonal spar andfar aft leadout placement in the Attacker wing, as I recall...
The problem with both was vertical weight distribution. Note the wart lifting the outboard wing in the second image of the ?purple? one? The wing was low, at the bottom of the fuselage. The upright engine's mounts were above the top of the wing. Made for a very small tank bay volume, too...
It needed more power than the ENYA 19 I used, and without the wart, the roll attitude in upright and inverted was so severely different that a good setting couldn't be found. The wart would probably cure that, but my model still seemed underpowered. Ted Snow flew an (?)Attacker at a VSC, with plenty of power. It did pretty well.
The pictures don't do it justice. It is a "cute" sporty-plane looking model, on the ground and from the handle. It would probably be much better balanced, vertically, with an inverted engine. That would allow a larger tank and reasonable vertical CG. ...OR, outboard leadouts with tip guides raised above the top of the wing... Either mod would be a long way from a decent representation of the originals.
Harry Williamson wrote several articles, mostly in MAN if I remember right. At least one article discussed vertical CG... Wonder what made him think about it...