Clifford Research http://www.cliffordperformance.com/Site04/Home.htm offers parts. Your engine is most likely a 153 CI but Chevy also offered a very similar 181 CI 4 cylinder that was also used in marine applications. The 153 was used in the Chevy II. The 183 is still in production. It is used in fork lifts, well pumps, generators, etc. The Mercury Marine head is superior to the one offered by Chevy. Kansas Racing Products http://www.kansasracingproducts.com/does a lot of work with these buggers and may be able to help you turbo charge. They may be able to identify your block and cubic inches from the casting numbers.