Do you have any rules about the engine to follow, or are you flying straight Open A rules?

There are several very high power, but high dollar .15 size engines, like the Cyclone and a bunch of others. They typcially run close to $200 each though.

Around here, the OS CV-A is used a lot, it's a bit expensive, but it's got good power, and is easy to run. It's not legal for SSC class though.

Norvel makes a couple of powerful .15 engines as well.

The Megatech .15 is on par with the CV-A, but a lot less expenseive. Quality control appears to be a bit hit-or-miss with it though.

Magnum makes a .15 that has good power, but the carb needs attention. If you look at the Magnum notes on my website, you can see how to fix up the carb to make it work right.

The OS LA is very popular in SSC, though it's not perfect either. The backplate can crack or loosen and leak air, and you have to remove the muffler baffle to get any power out of it.

There are other options as well, lots of .15 size engines out there.