Quoted from acebathound:

With the current state of things, if I was someone with soldering experience but didn't have the ability to make boards.. and had a massive Classic Bally/Stern collection.. I'd just solder the SIP resistors to the back of the board and be done with it (ie. http://img-f.pinside.com/201407/1754133/255254.jpg ). I'm not a "purist" kind of guy.. I mean I like seeing really nice looking older boards and would probably take those out and save them.. and modify a cruddier looking board, but there's no way I'd spend $50 extra per game if I had 10-20x games just to essentially add $1 in SIP resistors & wire per header ($3 per game). That said.. it's nice there's plug-and-play solutions for people that don't want to modify their boards to keep them original... or don't have soldering experience. I'm just saying what I'd do, with soldering experience and *that* problem to solve in the current state of things. If there was an LED solution that worked with Classic Bally/Stern games however.. I'd buy that. Or if the piggyback socket I mention above was something that could be done... should probably pursue that myself
That said, the plug-in boards are a solution between hacking your current board & buying a $100 Alltek right now. There's probably another solution out there.. it's just a matter of finding it