i have the trinity novarossi p5 and ive been extremely impressed so far
trinity is the exclusive importer for novarossi engines to the USA so if its a trinity its a true novarossi not a knock off
the ws7 2 is also a good choice ive seen plenty of guys running them who are very fast also
i dont care much for OS engines the piston and sleeves dont seem to last very long and they dont seem to get very good fuel economy but they do have a good warranty
if your track is large i would recommend the ws7 2 if its on the tighter side the p5 is the way to go
and i agree with kingzj about sirio so far i havent seen anyone have good luck with them except for the trinity sponsored guy who runs here sometimes and i noticed he was using novarossi carbs on them