First thing I would do is remove all the hoses from the pump and see if it runs up to speed. If it does, it's not the motor. If it doesn't it's most likely the brushes, easily and cheaply replaceable, if you know how to clean a commutator and reassemble the motor. Not complicated, but holding in the brushes when reinserting the armature can be finicky. Motors are easily and cheaply available on EBay. Those pressure switches rarely go bad, but they just screw into the base, so replacement is easy. I always carry spare valves, a belt, diaphragm and pressure switch. Lastly there is a rubber diaphragm on the end of the pushrod from the motor. A hole in that will pretty much disable the pump.
Since there are 2 valves in the pump (cheap and easy to replace) check to see if one is broken or has a piece of grit, hair, etc in it, because that will stop the flow. Take note of which valve is up or down (take pics as you disassemble). Check to see the bearing from motor to pushrod is turning easily; it could simply be a corroded bearing that is putting resistance into the drive system. If the valves are good and everything else checks out with the pump, move on to your water system.
I have half a dozen of those of various sizes and they are the best pump on the market. Since parts are available and they are so reliable (our bilge pump, 34600, is 40+ years old) and easy to fix, it would be madness to replace it with the new stuff with built in planned obsolescence. But if you decide to get a newer pump, please let me know as I'd like the old one. I have yet to get one from someone I couldn't repair for $100.00 or so, and most for under $30.00.