Several years ago I rescued a number of old 4:3 LCD monitors my employer was discarding, including several particularly nice 20" 1600x1200 Dell 2007 FP. My plan was to use them to create vertical monitor MAME cabinets. But having learned from past projects, I resisted doing anything on the hardware side until I'd figured out the software side. And then, like many of my projects, that's where it waited. I'd occasionally give the idea some thought, but never really do anything serious.
But more recently I've started giving trying to work out a plan. The basic idea is to use a Raspberry Pi Zero as the CPU, powered off the monitor USB ports. The monitor also has a 12V power port which could be used to power speakers - but that's hardware so something for later.
From a software side it appears there are a few different front ends which could serve. Basic idea is to select game via a very simple grid which plays attract mode videos. This appears to be doable.
But that brings up the question - what games? The current plan is for a minimalist control panel - joystick plus two or three buttons (depending upon what the games require).
I already have Lakka on an SD card for my Pi0w which has MAME 0.78 (aka mame2003). So I downloaded the Windows version, generated the XML file and filtered it by vertical monitor and original games. The plan is to then work through that list and try to figure out what works and what doesn't.
There are 700 games.... this might take a while...