The engine, afaik, is identical to a Picco P7R Evo 2 or 3(only difference between those two is the carb I think.. One has venturis, the other not???.. The LRP has venturis..)
I have the LRP's twin - the Nosram 21RR - and it's a great engine. The matching pipe is the 2063, but don't know if you can get that in the States.
Since it's a Picco, many would probably recommend an 086. I would try your JP-3 and see how it goes...
Runtime won't be as great as some of the newer other 21's. Power is there in heaps though... In the meantime there is a newer "Spec II" version, which is tamed a bit for smoother power and probably also better runtime...
Still, the original LRP ZR21.X is a powerhouse engine...