When I did my Ryobi and Homelite conversions I completed a long search of RCUniverse (and beyond) and settled on a Walbro WA-167. It will easily get a Ryobi or Homelite to 7000+ RPM provided you don't go crazy with oversized props (which some people are convinced MUST be fitted to every gasser).

OK, this is an example of underpropping but a Ryobi 31cc with a simple conversion (no porting etc) turns 8900RPM with a 16x8 MAS. Ideally, you want to keep under 8000RPM with a Ryobi or risk rod failures. An 18x6 or 18x8 prop should get you into a reliable rev range using a WA-167.

I only bench ran with the 16x8 as that was all I had available.....wouldn't recommend it for any extended running or flight.