I don't get it?  If all you did was add one standard player score display - the way I'm reading it is that's what you did - then it can't be a power supply issue.  Any power supply issue would have caused problems with all displays.  Or are you saying the added display stuffed the power supply? 

I have some WMS displays that don't have the keep alive voltage fed to them. They came as a bonus in a machine and the harnesses and wirring all look to be complete but the single keep alive wire is not fed to them in the usual way - it's just not there. I've always wondered but have not yet fired them up.

The display fuse is 250mA slow blow and I think the smart mod on the power supply is to drop the zener down to a 91 Volt to prolong display life.

Dropping the Zener in for +91 + -91VDC is a great idea - I do this to all power supplys I have to rebuild. It can sometimes highlight the weakest display with the reduced voltage, but it is better in the long run and easy to identify a display that will need to be replaced further down the track.