First, I must address the comparison of the Homey Vs. Ryobi. The Ryobi engine is about half a pound LIGHTER than a homelite. I have 3-4 Ryobi's, both short and long shafts, both 31 and 28cc's. I also have a 25cc and 30cc Homelite. The Homey's are by far the heaviest and worst engine to put on any aerobatic plane, if you leave the stock ignition on. If you trim away all excess metal, you can bring a 25 down so it almost matches the G23 for power and weight. The Ryobi's are lighter with the CH ignition, and more powerful, too--if you leave them all stock.
If you want to hop one of these up, the Homelites are the best choice--with the 30cc getting the edge. It's both lighter (slightly) and got more displacement than the 25. These both have a stronger crank and rod than the Ryobi. They also are piston timed, with better flow. The Ryobi has a weak reed valve that restrics flow a lot. See my web page for some hop-up tips, as others have suggested.
For the best bargain & best power/weight ratio, the Poulan 46cc is the way to go. See my site. When brand new, mine almost matched my $550 broken-in BME 44, and the muffler was way more restrictive. With a better muffler, I wouldn't be surprised if it beat the BME, or at least matched it. Weights were negligably different.
Having said all this, I think the new line of 1.8 and 2.1 gassers are better options for this plane. The H9 1/4 scale Cap is a nortorious PIG when it comes to weight. You should try to save every ounce you can. The RCS 1.8 and MVVS 2.1 are lighter than the aforementioned engines, and should have plenty of power to please. While I love my gassers, this particular plane is really too small for great performance using a converted engine. For the smaller conversions, stick to the floaters like a Cub or PT-19. If you want a larger one--like the Poulan 46, it's sized to go into a 27% plane, like a MW Cap or Extra, or a AW 29% Edge.
Anybody that wants a Ryobi 31 or Homey 30, I've got some extra's that come with the parts. I'll ship them for a C-note. The Homey has no ignition--either get a CH, or fly it with glow. The Ryobi, you get your pick. I've got more engines that I know what to do with.
Cliff
http://www.rcfaq.com