Porting a game from the C64 amounts to basically reprogramming the game entirely. It's feasible, but it's as long as programming a new game entirely. That may be the reason why we haven't seen a lot of those yet. The only thing you can really port realisticly from a C64 is the rules of the game or some internal calculations (like the AI). Even then, you have to reprogram it for the Z80 processor.
Also, lots of things on the C64 are fairly difficult to emulate on the Colecovision and, depending on what it is, it can even be impossible to do properly. The most obvious one is hardware scrolling. It can be done on the CV but lots of graphical details will be lost in the translation. There's also the hardware collision detection between sprites or sprites/background, which, for example, Space Taxi uses exclusively. 64k RAM vs 1k RAM, and the 32k "normal" cart limit are hurdles that can't be overcome in a lot of games (except by using special cart or the super module I guess).
By the way, I'm the one doing the Space Taxi Coleco game. I disassemble the original game just to get the original movement calculations routine, since this is one of the most important part to get the original game's feel. This is the only salvagable code from the original game though. The rest will be all redone. Also, if the game is released on a normal 32k cart with all 24 levels, I expect some things will have to be stripped down. Don't worry though, the talking passengers will stay in! I'll use the same routine as in the Squish'em Sam game, which Daniel Bienvenu worked his magic on it to make it even better (I think it was back in 2003...)!
Also, I thought about doing the Trolls & Tribulations game a while ago. The only real problem with this game is the hardware scrolling. But as the game has a mostly monotone feel in its presentation (mostly gray and blue), I think it's possible to make it look good on the Colecovision even with smooth scrolling.
Edited by tabachanker2, Wed May 7, 2014 6:18 PM.