Without seeing the machine. I had one that the light appeared square, but was an actual round bulb behind the mounting. And I was able to order a bulb at O'Riellys. I think it had a plate on the back side with some screws that you removed to get at the bulb. It is a PAR36 bulb, Wagner 4414.

 

Most lights like that have a 4.5" sealed beam that you can pop out and replace.

 

As noted above, I would check to see if it is a sealed beam.

 

Make sure you have power before buying a new headlamp.

 

Yes It is most likely a sealed beam that is available at most hardware stores. Canadian Tire carries them here in Canada. Make sure your getting a good ground. Many times the ground is the problem with these lights. You can pop the sealed unit out of the rubber housing and check the unit with a continuity meter to see if its burned out. No flow through the terminals it is burned.

 

Thank you all for your comments. I took a look at it again and it is just like the link I listed in my original post. There appears to be no access to the bulb and the entire thing appears to be replaceable for $10. The hardware store must have added it. When it was running I disconnected the main wire to the light and plugged in a volt meter and grounded the other end. I got no reading. I should have gotten 12V right? My guess is the light is fine and the coil that powers the light is bad or the wire is.

 

EVH,

What setting did you have the meter set to? AC or DC?

It's likely that an MTD from that era used a halogen sealed beam unit, that works off the stator current.
That would be AC however.

If you measured it as DC, you'll get a zero reading. On the AC setting, if you still don't get a reading, make sure and try a couple of different ground points, just to be sure, before you consider it dead.