About the Street Fighter II games...

[Gary_Oak]

How hard would it be to hack them in order to implement a combo system similar to that of the Smash Bros./Tales of series?

Something like this:

A = Normal Attack
B + Up = Shoryuuken
B + Left or Right = Hadouken
L or R = Guard
Up or (other face buttons) = Jump

[FAST6191]

At the risk of having something thrown at me what version of SF2? It matters a bit as reading through http://streetfighter.wikia.com/wiki/Street_Fighter_II it seems there are more differences than I first realised (and I knew there were some).

More generally it is an entirely reasonable hack to ask for (so very far from "can I run Halo on my NES" style questions) with an interesting premise, I certainly would not mind seeing a Street Fighter game with simple controls as it would change how it plays out quite a bit.
I am not sure I can quite call it standard control remapping (is this guard a timed guard and can I also time it right and get a counter or something*) but if you understand the reasoning behind it all then it is all the same ideas really.

*that said such a thing, if it is the case, would probably be in line with smash brothers anyway.