Here is what I figured out over a couple days of playing around. First thing, parts needed..720 reproduction controller connected to Ultimarc Optipac, 2 arcade buttons that can connect up to the Optipac.
I wanted to get this to work with a Raspberry Pi 3 running Retropie. I spent a lot of time learning the basics of the Pi. I could not get Retropie to recognize the 720 joystick in the game. I could use it to move the mouse cursor and the 2 buttons worked on Pixel desktop.
I know that the version of Mame is important. After Mame version .142 (i think) the 720 controller is supported. When you press the tab key to bring up the menu the newer versions of Mame have a "game configuration" menu that older versions do not have. In that menu you can select Joystick, Spinner, Real.. Select "Real", obviously if you have the real thing..
I decided to move to my windows PC at this point and ignore the Pi for now. I downloaded an older 2013 version of Mame, beacause the old PC I am using is dated. I read about editing the mame.ini file to edit the control defaults, but my Mame version did not have a mame.ini file. I downloaded and ran MaLa (sp?) front end for Mame, it created the needed mame.ini file because it did not find one. In that file it shows keyboard and other input values. My understanding is if it had a 0 the device was not enabled and if it shows 1 after it that controller type is enabled. I changed the mouse value from 0 to 1. That is all I did..after launching mame and loading 720 I mapped the buttons for kick/jump and now it worked. You have to disconnect your regular mouse or else it screws up the calibration. So it seems like just allowing mame to recognize and use multiple mice was all that is needed..
Now time to build a box. If anyone know how to edit .ini files in Retropie I would appreciate it. I tried lr-mame2014, it runs 720 fine. I found some online posts about how to access the mame.ini file but I could not really understand it. Retropie and Raspberry Pi people are much more technical than I am. I will keep tinkering with it because I am using a huge full size PC tower to run 720 now..would be nice to use the tiny Pi.. I could mount that inside my joystick box and have a portable arcade system. Maybe people on this forum may not too interested in this, but I wanted to share it so it may help someone else. Lots of retro arcade info online but not too much specifically about 720 and the unique joystick