PFA limit is 260 hp.
A quick search on the web will give you access to some American sites dedicated to converting just about any type of engine for use in a homebuilt, from V12s down. Bearing in mind their EAA rules are far less stringent then ours....
Designing a homebuilt from stratch is, a massive undertaking as you'll essentially need to work to JAR design codes etc etc, and PFA Engineering like to get involved soonest.
Far better to stick to a pre-approved PFA kit, think there is now nearly a couple of hundred to choose from.
I'm waiting to see if the RV-10 gets UK approval.... Unless of course your can design something with the performance of a Lancair IV-P !
Father Mulcahy