Finding the right board for a GPS tracker

Hello there!

TL;DR: What is the smallest board that can manage a GPS and a GSM/GPRS module, ideally also some output screen?

I'm new to these forums, great to meet you.

After my precious bicycle was stolen I'm now trying to figure out how to build a tracking device that, if this ever happens again, tells me where to find it. The general idea is to get the position via GPS and communicate it with a GSM/GPRS module. Ideally all the electronics are hidden in the frame of the bike or some other inconspicuous place, which makes size the most important factor. The part of the frame where the seat post sits has a diameter of ~28mm, which puts the simple solution (arduino uno + matching shield) out of the question.

To further complicate thing, I'd like to use an accelerometer for activating and have a display to read speed and traveled distance.

Which controller board would you recommend under these circumstances? If there are any that fit at all.

If you'd like to know more, I plan to tie in my generator hub for power supply, of course along with a rechargeable battery. The generator puts out 6V at 3W, but is of course also powering the lights. Once I figured all that out I might even have the bike collect tracking data whenever it is moved and send it right into the web. My first estimate is that I could send 600 sets of position data for 0,01€ if I send them in bulk, so why bother with physical data transfer?

Finding a small size GSM board turned out to be challenging, but for now I'm looking at this one (the connector for the antenna will likely get in the way, but that's something I can work with):

Something that I still need a good solution for are the antennas. The GPS could possibly be hidden in a light or reflector, those thing are small and need sight of the sky. The GSM antenna will be trickier and for what I know it won't find any signal inside a metal tube ;)

Project costs have to be reasonable, of course, I'm thinking of no more than 150€ for all the parts. That's quite a bit for a bike worth 600, but if it doubles as a speedometer and trip recorder that saves a bit on other electronics I'd want.

If you have any ideas or recommendations to add I'm happy to read them. While I have a little experience from building a walking robot, I'm getting kind of ambitious with this project. If it ever leaves the planing stage, that is. Thank you for reading!