I've run a couple of Maculloch 28cc weedwhacker conversions pretty extensively. It's a good engine for planes up to about 14~15 pounds and I'm going to say that you'll want a good 1500 sq inches of wing area.
My Macullochs love a 16x8 prop and will fly something like a quarter scale clipped wing cabin plane very realistically. I had the first one on a quarter scale Swick Taylorcraft and the second one went on my old Phaeton 90 bipe. You can see photos of both planes (RIP) on my webpages.