Ok - here's my attempt to help you without casuing too much major work/redesign. You have a lot of potential here. Some of these things have been mentioned, but I will repeat them anyway.
1. The gray cabinet if fine, and with the black t-molding, looks pretty good. Don't worry about repainting it. The Data East sideart is in pretty good condition. This cab looks better than an all-black cab without side art.
2. Curved marquee - this looks great! Just get a nice marquee printed and make sure the thing lights up. This will change the appearance nicely.
3. Speaker plate above monitor - finish it :-)
Put a piece of wood in there and mount the speakers...easy.
4. Remount the monitor. This will be the second most difficult thing I suggest, but it will be huge in terms of appearance. Mount the monitor about 1 inch deeper into the cab so that you can fit a nice piece of bronze plexi in front of the monitor.
THIS IS IMPORTANT. Combined with the speaker plate, it will seal up the gaping holes and make the cab *look* finished.
5. WARNING - this is the part that might suck.
Completely redo the control panel. It looks terrible. If you had a 4 player cab, that CP *might* look *ok*
There is way too much acreage on that thing for the cab. Make the CP as wide, or maybe 2 inches wider than the cab. Don't have it hang out more than 3-4 inches past the front of the cab either.
This is very important as it will bring your cab back into the "could be a real arcade machine" instead of "this is my first MAME cab"
6. Find the missing pieces to the coin door (check happ controls), or find another coin door that isn't missing pieces. It makes the cab look incomplete. You want this thing to look like it could actually be in an arcade without feeling out of place.
7. Put the back panel on. If you don't have it, make on. The light shining through the back will look weird when it is all done if you don't close it off.
8. Enjoy your newly finished, great looking Data East to MAME conversion.
You are almost there, you just needed a little motivation.
Good luck,
Aaron