225 rebuild question...

I bought my son a car and we got it running, one of the rods keeps slipping, my borther said it was something about a valve sticking... we are going to rebuild the whole thing just be cause we do not know what is in there and there is a lot a build up. This is my first time doing anything like this and i have people looking over my shoulder to help me and my son do this together. My question is, where is the best place to get complete rebuild kits for a 225 out of a 65 dart? Thanks