Erik is correct to say that there are several 'drop in' replacements with IAP.

However, when enhancing or removing bugs from firmware in old "collectors items" kit, it is often the case that much of the value of the item lies in the fact that the internal parts are in 'original' condition.

Sticking in a new MCU might well devalue the goods. You might even argue that removing 'historic' bugs might have this effect (even if it makes the game more playable).

Derek