Similar thoughts here. If the cabinet is in bad shape, the controls are no good and the monitor is suspect, You've probably paid a 'full machine' price for essentially the only really transferrable item left working - the PCB. As i's either a case of buying yet another JAMMA-wired shell and components to house a multi-game board, you may do better to cut your losses, sell the board and remaining components as seperate parts and scratch-build a PC-based cabinet or at least do a PC conversion on a gutted cabinet, if you want a multi-game system.
If however you want to continue as you are, the best suppliers are guys like Ultimarc, GroovyGameGear and the like for controls, though please don't underestimate the cost as some noobs do - a 2-player panel with a suitable interface and wiring will run you between £100-£150 for a 'fighting' 6-button layout. There are cheaper chinese parts available on places like ebay, as well as alternative interfaces, but without getting all 'purist' on you I can say that it is the controls that will make or break an arcade machine so you should never scrimp on the controls. If you are looking to save money, do so elsewhere.