Hey Cwalters the engines are definately wortha try.

I am not familiar with the 421b but I did run a P5X before the Go's and if you like that kind of power I would say try the Go Tech 5port Race engine Trubo. It was very similar power compared to the p5x but with more linear powerband and more on top.

The Turbo plug does have a major boost in performance, mileage and power is increased and turbo plugs last longer. For standard I would reccomend Go #5 or O.S. #8, for Turbo I would say O.S. p3 and if your on a budget O'donnel 77T or 97T

The Go tech uses a 4bolt head pattern and is compatible with any Novarossi 4bolt head.