Hi John
Just build it as per the plan. I've built 4 of them in my time and used various engines ranging from a Zenoah 20 to a 160 OS Gemini, OS 120 Gemini, a OS 91 4st (marginal on power for taking off from grass) If you're feeling flush it goes without saying that the Gemini 160 is the engine for it (just for the scale looks and sound) but any 120 4 st or a small petrol will be fine but if using a petrol you'll end up having to cut half the cowl away
Hope that helps
Styk