I posted flipper suggestions on the pinwiki:
http://www.pinwiki.com/wiki/index.php?title=Sentinel_/_Wonder_Wizard
Mods I would do to Sentinel/Wonder Wizard machines are:
Remove those springs around the slingshots and install real slingshot switches and rubbers.
Remove the metal contact rings around the pop bumpers and install normal pop bumper skirts and spoon switches.
Beef up the bridge rectifiers on the motherboard from the stock 2 amp units to either 8 amp or 10 amp units.
Split the ganged slingshot fired by one coil into the usual two slingshots. One could still be operated by the game board while the second one could be operated by a Gottlieb System 80 pop bumper driver board.
Split the ganged pop bumpers into individually fired normal ones by operating a couple of them from Gottlieb System 80 pop bumper driver boards.
Install a normal Williams or Data East ball shooter assembly.
Add white LED's to the backbox lighting circuit.
Add more lights to the playfield. Remember, the power transformer does not have much headroom electrically to add very many incandescent light bulbs, but LED's on the playfield should be ok.
Get rid of that phony "ball-and-chain" tilt assembly and replace it with the usual plumb bob tilt assembly.
Add a playfield prop stick to the cabinet.