I am looking at building a CR500AF. I am possibly buying everything from a guy who was going to start one, but then decided not to. The one thing I am curious about is that I have never seen an AF with a motor older than 89. I am wondering if I am still able to use the 88 motor, but use an 89 exhaust? It would fit better into an aluminum frame. I would be using a 2009 CRF250R frame.