OK - so I gather you have bought a non working project game. You mention it is "cheap", so you got what you paid for ?

I'd start by downloading the manual - http://www.ipdb.org/machine.cgi?gid=738 All the part numbers you need will be in here.

Mike has the flipper opto assemblies, and I'm sure Greg at RTBB has all the parts you will need to get the machine in working order.