water pump pulley question

You are on the right track going with the smaller pump pulley. See here:

www.stewartcomponents.com

Not sure what model you have but many were made when leaded petrol was available. The lead helped cool the combustion process.
Smaller pulley. Depends how creative you are & what tools you have. I have many times gone to wrecking yards to find a pulley of the diam I want. I then machine off the pulley section & mate it a factory pulley by machining/welding. Simple job for a machine shop.