If you compiled the BT5 kernel and ran it, you would no longer be running Fremantle at all, you would be running BackTrack.

There's technically nothing stopping you from doing it, other than the process itself. I would not recommend doing it if you're trying to accomplish something practical. If you're just trying to learn and do it for the experience, sure, give it a shot, though I would suggest starting to learn kernel compiling with an easier target than this.

As for the other tools,nearly all of them are open source and downloadable, so sure, you can give it a shot. It's a lot easier said than done to get dependencies working in Maemo though, as it has been years since many of them were updated. A lot of people prefer to do compiling in a Debian chroot or now a BackTrack chroot for that reason.

Basically, if you just want to do it for funsies and the experience, knock yourself out. If you're trying to build your own better version though, remember that BT5 took a whole funded team of really smart guys quite some time to get this out. If you don't have resources of your own, this is going to be harder than you think.