NIB?, that's almost 70 year old. I'm surprised it ran that good. Mine is hard to get it running intermittently. I would run it from a switch, it seems it needs to "warmup".
If you like to tinker and are good at it, I would have a go at it. And it may take lots of patience, trial and error. It may get worse before it gets better. I have more than one of these vibratory motors and know from experience. When you are all tinkered out you may have to get new parts. The Lionel replacement string may work, but I went through 4 different strings weights and weaves to get the right combination on one of these devices.
First you need to reduce the force required to move the linkages and figures so the motor doesn't have to work hard.. Look for any joints and lightly lube them and make sure there is no mold flashing or other debris. Of course do not lube the vibrating motor parts, like the pulley and string. When it works, its great but its a balancing act between the spring force and pulley friction, and string. You can try and disconnect the fixed end of the spring and hold it with something sharp so you can manually change the spring force while its in operation. You might get lucking a notice a sweet spot of operation, then you can modify the spring for permanent attachment. I got mine working at 13-14V and with trains running I don't hear it.
Some folks recommend roughening the pulley groove so the string has more bite.(friction). This might help but I've had instances when it didn't, Remember this is an irreversible operation.
Good luck.
