Blockhed44 - I know what you are saying. You just want to do it yourself and upgrade, but everyone is telling you its a bad idea, instead of helping you. I actually did upgrade a Harvard table. First, I dropped out the flimsy playfield, glued 4 sheets of MDF under it, and bolted the whole thing back in. That goes a long way to making it a better table. You need a flat, level surface, not a bowl, like the stock Harvard has.
Next, I hung metal plates on wires, at 45 degree angles, in the back of the goalie boxes. Tornado tables have a loose plate that absorbs the hit of the ball when you shoot hard, so it does not bounce back out, like it does on the Harvard. My hanging plates worked like a charm AND had the added benefit or ringing like hell when you scored on somebody
I also have experience drilling out men. I sell a foosball trainer, and I recently made a video to show it works on a Warrior table. Well, I didn't HAVE a Warrior table, so I had to get Warrior men and drill them out. I have a full shop and am quite experienced at building/fixing stuff, but they did not come out great. It is REALLY hard to drill a hole bigger, once it is already there, because your drill bit has no place to stick the tip, and keep it centered. It worked fine for my video, but there is NO WAY I would play with them. Even after pinning, they are loose and rattly.
OK, now is the part where I tell you those other guys are right
I hate when people tell me I can't do something, or shouldn't. I rarely do things the way other people do, and often I come up with better solutions. I sense you are just like me, in that regard. Having said that, I eventually sold my Harvard and bought two Tornados. Why? Even with all the upgrades, there is just no comparison.
To be fair, if you are just whacking at the ball, which is really fun, there is not that much difference. The guys is this forum don't do that. They all precisely control the ball, pass it from rod to rod, and execute planned shots. Even with an upgraded Harvard table, forget it. The Harvard is a toy and the Tornado is a finely honed tool.
Also, the Tornado is the standard. If you and your son want to go out and play anyone else, they will all be playing on Tornados.
I think it is AWESOME that you are playing foosball with your son. Don't stop. If you want him to be able to grow up and kick ass on a public table, Get a Tornado. These guys will argue with me on this point, but you could probably get by with a Warrior or Fireball.
Good luck,
FoosTrain