The O.S .18cvr has some insane power. Very easy to tune and very reliable, and holds that tune like you wouldnt believe . The .18cvr should drop in without a whole bunch of mods and would be well worth it. And it's only $109.99 for the pull start version. Or if you dont want that much power the .12cvr has a ton of power but without as much torque. The torque on the .18cvr will have you doing wheelies without a problem. The .12 does not have as much torque and you will be a little hard pressed to find wheelie's but it still screams and is a very good choice. That engine will cost about the same maybe a little more then the .18cvr. But I would definately get a new engine though.