I think you should consider another possibility, that there is blockage down stream of the pump and it is trying to pump against too much pressure. I believe that if the intake were restricted you'd see overheating of the impellor rubber before it would shear from the hub. Whereas high back pressure would likely show up as shearing. Is there a pressure guage, or guage port? If not I'd consider drilling and tapping a pipe thread hole for one.
An old engine, an old muffler, or old piping and hoses could have lots of sediment in them. A new engine might have casting defect, like a core that didn't get properly removed, or a core that was broken during casting and left only a tiny passage. Sometimes they forget to drill out a passage to the proper size.
A pressure gauge will answer lots of questions quickly. I like the test ports from Sisco P/T Plugs - Pressure & Temperature Test Plugs
You use one gauge with multiple ports. The gauge is easily checked and seldom goes bad because you don't leave it connected. When working with centrifugal pumps you measure differential pressure using the same gauge, so gauge error is already accounted for. I buy these things by the dozen!
Gary H. Lucas