Know this is old, but if you're like me, sometimes projects get shelved a while.
That converter you're using had a PDF online I glanced at. It stated:
"Question: Screen has black areas, or the screen is very dark to almost black.
Answer: Adjust the "Clamp sp" and "Clamp st" settings. Keep the "Clamp st" lower than "Clamp sp"
Also, when I installed an ArcadeVGA card into the PC for my MAME cabinet, I seem to recall it states that some arcade monitors would need the signal boosted. I got lucky using a WG K7400, and it supposedly takes as low as 1 Volt video input signal.
I believe ALL Wells Gardner monitors are supposed to be able to accept this low of a signal, so I wouldn't think this is your problem. BUT, I couldn't find the output signal power of the converter you're using, so you could look into this product if you think the signal needs boosting:
https://www.ultimarc.com/arcade-vga-and-video/arcadevga-accessories/arcade-monitor-video-amplifier/Also, that white gunk glopped at the bottom of your capacitors holding them to the PCB is from the factory to strengthen the connection. If it's unbroken (hard to see in your pic) it's basically a tell-tale sign your chassis has NEVER been recapped before, as most people would simply scrape it off when they install the new caps. So, it's not leaking electrolyte.
Good luck.