French bikes are surprisingly easy to deal with and the UO 10 does have a lot of room for fat tires. It's easy enough to spread the rear triangle. I'd overhaul the bike first to make sure the bottom bracket and headset are fine. There are replacements out there but hopefully they are in good shape. Replacement parts are available for both BB and HS.
The bike came with a good quality crank, a stronglight 99 with an 86 bcd crank. That is basically a compact crank and chainrings are available. Spa cycles in the UK sells them or you can find them used.
No big deal replacing the stem and handlebar if you need to.
There is no integrated hanger so you will need to run a RD on a claw. The long cage simplex that came stock isn't bad but the best vintage rear derailleur would be a suntour VGT.
I'd replace the existing brake shoes with kool stops.
Fun project.