Do you have any binding of the the motor's drive shaft (armature) by tight bearings?? How about the bearings for the shaft on the driven gear that actually moves the reciprocating arm that moves the cable rack for the wheel boxes??? If not, you may have an internal electrical problem.
The motor used is a permanent magnet motor, meaning that it does not use a field coil to create an outer magnet pole for the armature to work against to provide rotation. The armature is working against a permanent magnetic field. Two things could cause the motor to have less power (torque) in this case. One would be a problem in the armature where it would not be capable of generating a strong enough magnetic field of its own to work against the permanent magnet's field. The other problem could be that somehow the permanent magnets have become weakened enough over time to have lost some of their magnetism, enough so that the motor no longer provides enough torque. Either is a possibility on a car that has been around for many years. Exposure to heat or alternating currents can weaken the magnets, and open or shorted wiring in the coils of the armature can mean that only some of them are working when they should and torque is lost since a consistently strong magnetic field is not being generated at the armature.
Testing the coils in the armature is easier to do than to judge the strength of the permanent magnets. You would have to have a known good set of permanent magnets available to test for their strength vs. what you have. The armature coils, however, could be tested with an ohm meter for continuity and impedance.
As you have probably noticed, the brushes are set at three different positions. One brush is considered "fixed" and is the return for either speed. One of the other two is directly across from it, and the other, at a different position. By placing the leads of an ohm meter at the corresponding points on the commutator (that area on the armature where the brushes normally set) as it is turned, you should be able to see continuity and some sort of impedance (resistance in ohms) between each of those two points as you work the armature around. If you find a set of contact points on the commutator that is open, then you have probably found your problem. If not, and all such points test good, then I would suspect that your magnets are weak. In either case, you are looking at buying a new motor unless you have access to one you can cannibalize to make one good one from two.
If you are planning to keep your MGB for a while and do your own electrical work, I would recommend that you consider getting a copy of Rick Astley's book, "MGB Electrical Systems". It is by far the most complete book on the subject and written by an enthusiast who is also an actual automotive electrical engineer.