If you place another battery in the circuit, and there is no change in spin rate, either the wiring to the starter is "weak" or the starter itself is weak. There could still be a bad ground.
To test you glow plugs, you can test them via an GOOD VOM, their resistance is very low.....you can also hit the glow plug switch and see the spot in head increase with a Digital Infrared Thermometer - it might take 30 seconds or more.
You can test the starter by bypassing all the wiring but ground.
Make sure you are in N and have someone sit in the seat with foot on clutch/brake what ever it has.....
Take one end of your jumper cable and place it right on the starter pos. post, hook the negatives up, and then place the remaining hot to the known good battery.....
if there is a change in the starter behavior then you have found your problem (wiring), if it still spins slow - I would suspect a starter problem.