Welcome to VE!
I'm not sure exactly on your issue keeping it running. It may be your motor but I would suspect that there is some old, dried-up grease in the mechanism that isn't allowing some lever / cam to operate properly ... probably something that actuates a micro-switch to start / stop the turntable. Perhaps a lever that is sticking and making the turntable think it is at the end of the record. Of course it all may be completely out of adjustment too. I suggest you take the bottom off and prop the turntable up on something so you can observe what's happening during the start-up cycle (I use a pair of speaker stands to do this). Once you get an idea of how it all works ... or an idea of perhaps what isn't working ... a cleaning and re-greasing of the mechanism would be your 1st task. You can clean old grease with isopropyl alcohol. Use a light, plastic compatible grease. I use Labelle 106 which is sold through hobby shops (primarily for model trains) or online. Lighter than a standard lithium grease you would get through your hardware store (you don't really need or want a grease that works on your trailer bearings!) and plastic compatible.
At the same time, since you have everything open, I'd pull the center spindle (look for a set-screw in the side of the bearing well), clean out the old oil with alcohol (probably black sludge by now) and re-oil it with a light oil (sewing machine oil works). I'm not sure if these turntables have a ball bearing in the spindle ... so careful if it does not to lose it.
The motor may have an oiling port on it (look from the top next to the motor spindle for a small hole, sometimes marked 'oil'). Same light oil will work.
Clean any old belt goo / residue off your motor pulley and belt path on the platter as well. I use a metal polish like Autosol or Peek to clean both followed by alcohol to remove any residue (the brass pulley will gleam like new!)
The mechanism cleaning would be the most important to do and may or may not solve your issue but the clean-up is all turntable-service-101 stuff.
There are a number of Yamaha models that I believe were identical except for finish, but were Canada-only versions. So yes I expect the YP-77 is the Canadian version of the YP-B4. Canadian versions had a rosewood finish and black accents rather than walnut and silver accents. My 1st amp was a CA-44 - black face, rosewood & 25 wpc. The CA-410 (Mk1) was silver face, walnut & 25 wpc and a dead-ringer.
By the way, somewhere in my 'to-do' stack I do have a YP-B4. The one I have has had its auto-mechanism stripped out of it and an external start-stop switch mounted on the side of the plinth. It's not worth trying to put it back to the way it should be, but obviously the previous owner had some significant difficulties with it ... who knows perhaps similar to yours? Let's hope the fix isn't as drastic! Anyway, I was thinking it would make a cool wall-mount turntable clock. I'm pretty sure the motor works on it so if you do feel that the motor is a problem I could probably Help you out in that respect.