If you're running RetroPie, it's most likely using an unofficial hacked version of MAME that is so old it doesn't even support the PE+ system at all (case in point, the most common fork of MAME in RetroArch is mame2003, which is based on 0.78 - the initial PE+ driver was added several years later in version 0.119u4). Even mame2010 (0.139u1) isn't really that good for PE+, the bill acceptor isn't supported and it takes an eternity to coin up (I should know, I was playing them back in 2010 itself when I was helping work on the Aristocrat 540 driver - the MK2.5/MK4 games only started doing useful stuff in 0.139u2).
You will need a modern version of MAME (e.g. a version made in the past 5 years) and a Pi 4, although you might get away with using a Pi 3 due to the PE+ being a basic 8-bit machine.
You are far better off just picking up an old unwanted PC like a Core 2 Duo/i3/i5 as it will be far more powerful and allow you to run a lot more things with ease.