There probably isn't a good troubleshooting guide.
I would undo the mod and get it working first. You can use RCA->coax adapters with old systems like that in place of the tv switchbox. That does away with the need for a mod if you have a tv with coax in. On these really old ones this is the order I usually follow:
1) Google
2) Replace all electrolytics
3) Reflow solder in the questionable circuit
4) Shotgun the questionable circuit
5) Deem it a parts machine