The only thing that should need to be changed is the coin mechanism itself, and maybe the slot. Maybe. Here is what happens when you put a coin in the coin slot of a game:
1) Coin goes into slot, rolls down chute into coin mech, which will determine if the coin is "good" or not.
2)If it is not good, the coin is routed to the coin return cup.
OR
2) If the coin IS good, then it will route the coin through a "gate" switch, which tells the game that a coin has been added. The coin will then fall into the cash box.
The great thing is that the coin mechanism itself is easily removed, and they've used the same design for the mechanism itself since at least the 1960's. Therefore nearly any coin mechanism made in the last 50 years should work. Ones for pinball or video, anyway. Fruit machines, vendors, laundromats (I think you call them "wash-blubby" or something across the pond), jukeboxes, etc may have different mechanisms. It sounds like you're on the right track.
Now, if you REALLY want to go nuts, I've heard of people modifying existing coin mechanisms to accept different widths and sizes of coin, where necessary. There are some threads about this over at the KLOV forums. I have even heard of people removing the coin mech entirely, replacing it with aluminum u-channel in such a way that the coin will not be subject to any kind of slug-detection ritual. In this case, ANY coin will start the game, since the u-channel will guide all coins through the credit switch and into the coin box.