I use a JPAC. My cabinet doesn't use jamma either... I ended up just soldering wires to the board. I had one lying around so...
HDMI-VGA adapter plus VGA-CGA adapter - may be fiddly to set up and the lowest quality option. A lot of parts and conversions.
I had an HDMI-CGA adapter (or something like that). Several of them. Very disappointing. They had a lot of overscan, with settings to remove the overscan... but those settings didn't persist after a restart. Total crap. One of the 2 stopped working quickly, and I don't know if the second ever worked. Cheaply made junk imports. The ones that aren't are marketed cheaply go for $250, and probably have the same hardware
I had a Gert VGA. I prefer using the Pimoroni Arcade X-Hat on the GPIO pins for cabinet functions... so I hate that the Gert takes up the GPIO. Besides, I think these days you can pipe out the same thing via composite without using the Gert.
All I know is that after fiddling with a good solution for a long time, I plugged in the Jpac and forgot about it. Done. Easy. There are probably some resolution purists that would go on about HDMI-VGA-JPAC but... if that's the hang up, don't buy a Pi to begin with. I can get a screenshot of that machine running here in a bit.