I bought the 3 hp version of this compressor in 77 or 78. The pump pulley came loose about a month ago and destroyed the end of the crankshaft. Didn't worry too much about it though as it had been taking longer and longer to build pressure over the last year or so.
There are some parts available for these pumps, but they're priced like gold. I wanted to fix the compressor though, as the tank, motor, etc is good and I still use it regularly even though I've got a 5 hp 80 gal Quincy in my detached garage. No interest in rebuilding the old pump as that would've cost 4x what the compressor is worth.
So, I bought the 3 hp HF pump. It has a larger pulley than the Sears pump, and is made for an A section belt rather than the original poly v. It's designed to run at 1050 RPM, so, with a 3450 motor, the motor pulley needs to be 3.75" diameter. Tractor supply has a 4" pulley that will take either an A or a B section belt. With the A belt obviously sitting lower in the groove, the pulley is effectively 3 5/8 dia, or about as close as you'll get without going to an adjustable pitch sheave. The TS pulley has stamped steel sides and a machined steel hub so it will hold up far better than a die cast one.
You do have to drill new mounting holes in the plate to mount the HF pump, but positioning the pump so that it will align with the motor shaft without relocating the motor is easy enough.
So far, I'm impressed with the HF pump. It has a decent air filter and is much quieter than the original. I've got it set to cycle between 95 and 125 psi, and it will consistently refill the 20 gal tank and shut off in 25 seconds. When you do the calculation for CFM, it's doing about 12.5 CFM at an average pressure of 105 psi, which is significantly better than HF's claimed performance. I changed the oil after about an hour of runtime, and saw no evidence of leftover trash from the casting or machining operations.
You do have to bend up a new discharge line from the pump to the tank connection, but the HF pump is set up to use 1/2" OD copper tube, same as the Sears pump.
Cost was $140 for the pump (no 20% coupons on the pump), $20 for the motor pulley, $15 for the v belt, and maybe $10 max for miscellaneous fittings, fasteners, etc. I might be able to modify and use the original belt guard, or else fab up a new one, but I'll take care of that once a couple more pressing projects are out of the way.