PLEASE!, stay away from the Ofna Force .12. They are JUNK. Had one and barely got one gallon through it. Took it apart and it looked brand new inside, but it just wouldn't keep running.
Go to the HPI website and look for the "Buy HPI" and then the "Speed Shop" pages. They have a ABC piston and sleeve set for less than a new engine. This will make your engine better than any of those CHEAPO engines you listed and for less $$$ too.