Fox engines are still being manufactured today, as is Fox fuel too.
With lapped pistons you need to use castor oil, more modern lapped piston engines can do well using a blend of synthetic and castor oil, but are best with 100% castor oil. For a new lapped piston engine you need 25% oil, but it can be reduced in a broken in engine. Old timey engines can use upwards of 33% castor oil in them, with 25% being the minimum. A old well used mapped piston engine needs 25% castor oil at the minimum and you do not clean the piston and cylinder either as the old varnish is actually helping improve the compression. If you ran a fuel with 100% synthetic oil through a old varnished up lapped piston engine, it will remove the vanish and the engine will loose all of its compression and you can't put the varnish back on by then, it is too late.

I think the little HB .15 engine was one of the sweetest engines in its time. A modern .15 runs circles around them, but the HB .15's were just such gooid runners though.

Cool Power fuel is Ok for use in a modern plated cylinder ball bearing engine, but never use it in a older lapped piston bushed crankshaft engine, it will quickly destroy the engine.
You likely ruined the little HB .15 doing that.