This afternoon I fitted the Suntour derailleur. Cabled it up, adjusted, and it downshifts perfectly - but the gap and size difference between gears changing up are simply too much for it. Exacerbated by the fact there are no cable stops.
So after due diligence - it's now running single speed on the middle cog. I'm happy with this too, Rat-a-tat dontchaknow.
Tension adjustment works with the horizontal dropouts.
The other impediment to gear changing is the axle is slightly bent (I think) - because the freewheel does the orbital thing worse than just being threaded on.
But this will work Jim Dandy. But I might get a bling-er chain.
Another upshot is I would have put the decals in the wrong spot If I persisted It's gonna be scary fun for a while.
I might try and restore the Simplex derailleur down the track - but I still don't think it would work.