You have an air-cooled PG that was the downfall in the postal vans as they overheated in all the stop and go situations in cities, they worked great in rural routes. If you search the midget racing forums and want ads you can find all kinds of go fast pieces for the 153's. The newer 181's (Mercruiser) use an 8 port head that is readily available on EBay and marine repair shops.
