First off, I'm not a programmer but I've done some research on this.
This is the Freeware Spelunky:
The Original Spelunky seems to be modifyable and it would be interesting to see this get ported to a Raspberry Pi.
1. The Game is Freeware and can be modded and freely distributed.
2. It uses Mark Overman's Game Maker: Studio: http://www.yoyogames.com/studio
3. It's Source Code is availible to download:
If anyone is able to port or recompile this for Raspberry Pi it would be awesome. I just thought I share this as I've looked on the forums and haven't found anyone who has been able to do this yet. I completely understand if a person would be unwilling to port this to Pi but I feel having the information here would be helpful to those interested in doing a Pi Project on this. It would be cool if a mod of this can be put on the PiStore for Free or if it can be compiled and put onto Raspbian Repo.