Probably whatever was available at the time
People always ask what the original monitor was in various games. The truth is that it doesn't matter. Many games were shipped with different models of monitors throughout their production run. Also, when these games were in service, the monitors got swapped around a lot. Picture yourself as an operator in the 80's - you have a working Time Pilot out in the arcade that's bringing in good money, until the monitor dies. You've got a Spy Hunter in the back room that's broken again - but it has a working monitor, so you swap them in half an hour and get Time Pilot fixed. A couple weeks later, you have spare time and fix the original Time Pilot monitor and put it back in the Spy Hunter, fix the power supply in Spy Hunter, and put that back out.
All the standard 19" raster arcade monitors are pretty much interchangeable. They come in two basic mounts (horizontal and vertical). Many vertical games use "horizontal" mount monitors, mounted sideways (Pac-Man, Tron, etc). All you need to do is to determine what style mounting frame your game uses, find an appropriate monitor, bolt it in and hook it up.
So, what this means is, that even if you determine that Time Pilots all shipped from the factory with, say, WG 4600's or something - there's no garuntee that's what is in there now. You'll have to look for yourself.
-Ian