If it is a Grundfos JetpaQ water pump, there are several sources on the internet for the pumps and potentially for the parts.
You might take a look at:
http://www.marineparts.com/partspage...and/grund1.htm or
http://www.awqinc.com/jetpaq.htm
There is a manual for the combined pump/accumulator model which is likely the one you have indicated is too large at
http://tinyurl.com/qcjps that includes a troubleshooting guide.
Have you looked at the model GP JP-5 which has an adjustable 30-50 psi pressure switch and does not have the integral accumulator tank?
The manufacturer's address is:
GRUNDFOS Pumps Corporation • 2555 Clovis Avenue • Clovis, CA 93612
It may be that the particular model you have has been discontinued but if you have been pleased with it, it might be worth contacting the manufacturer about repair/parts.
The Headhunter is a good pump and is currently available but sources for either product is limited and prices are high as they are primarily used on yachts.