Those heads are pretty messed up!
@Eric (McGruff) said-"I'm guessing whoever ported and polished the head took out too much material behind the valve seat."
Or the press fit was wrong. How high does the engine rev with power? I'm guessing that with the added effective duration the 1.25's provide it must peak up around 6,000 rpm?
If you want a big kick in power- up the compression to 8 3/4 - 9:1. You'll have to run premium, but it's a toy, right? And you'll be amazed at the difference in throttle response! If you're insistent on running regular you can still raise the compression 1/2-3/4 of a point safely. Again, you'll find an improvement in throttle response and should feel an increase in low end and lower mid range torque, which will translate into slightly better mileage (if you can keep your foot out of it).
Hope this helps. Al
I take it you're going for the Panchitos with the cnc'd combustion chambers?