81 rabbit diesel truck power loss, injection pump?

I bought an 81 VW diesel truck and it just doesn't have the power or get the gas mileage that my old one had. It gets about 30 mpg's and has 75% of the power of my old one. It'll barely do 65 mph. The compression is good. I thought it was coking of the injectors but I've put fuel injector cleaner in it for the last 2000 miles and nothing's changed. I think it's probably the injector pump, but I'm not sure. There is a new engine in it and the bosch pump doesn't seem to be the right one for that year. I can't find one on
car-part.com. Should I have the injection pump rebuilt or try to find a used one? I can't find anybody who knows anything about it. What are some good websites for parts or info?

Thanks

by new engine do you mean tdi or a rebuilt 1.6? check the air filter, if i is very dirty it will affect milage. try having the injection timing and valve lash checked as well.