Class 10 just had some engine rule changes. Go to the Score website and they have a page explaining it. I know a lot of people are using Honda engines ( V-Tech ) out of Del Sol's and they seem to be very reliable and have plenty of power. Toyota also is a choice of many. Danny Anderson used a Toyota motor last year with lots of success. I will find the page and post it in a few minutes.
Also, if you are looking at getting a class 10 motor, often times you can find them for sale in Dusty Times in the classifieds. Sometimes they are new and sometimes they are used, but I am sure it would save you money getting one from there. A lot of times those motors for sale in there are race proven, and the owners are just selling the motor because they sold the car as a roller chassis.