You have alot of good engine recommendations here.. I will add you might consider the NovaRossi N21BF. $150 from a main, and gets rave reviews everywhere. The Werks B5 would also be a good choice, from the reveiws..

I was set on getting the Trinty ExTech II .24, but have decided to pick up two N21BF engines instead.. They are said to be excellent engines on medium to small tracks.. Big open desgins need more top end then these 3port engines can deliver.. That's where the Werks B5 would be a better choice, or a NovaRossi plus+4.