Anyone with the talent required to transform Skywyre to a sidescrolling engine will find everything else to be a piece of cake. Don't get caught up with who did what and when. Worry about who can do that and now.
That is a very good, and reasonably fair offer for all the work required but please, please please consider my recommendations below:
1. Setup a bounty/price for each feature. Go one at a time.
2. Don't pay a dime until you see a compiled, working copy that you can test for bugs. Thoroughly test it.
3. Only after testing and making sure it works, send payment for a feature.
I personally have been on both ends of the game, paying for code and coding for pay and I never had a problem on this site. That said, others have been scammed and the admins here don't have the power to help if that happens. - Be safe with your money! That large offer is bound to land a ton of PMs in your inbox!