How do I glue my magnet back on

The key to good adhesion is geting the mating surfaces clean, very clean. Do that with whatever means is required, steel wool maybe, solvents like alcohol or lacquer thinner may help. Just don't get sloppy and spill solvents where they may delaminate something. Maximum adhesion in most instances is best achieved with a slow cure epoxy. A common, easily sourced one is "JB Weld." I'm fond of marine bonding epoxy adhesives, but they're harder to find.

Most lacquer thinners contain aliphatic and/or aromatic solvents which will leave a hydrocarbon residue. I would use denatured alcohol or acetone to clean the surface but be aware that both have low flash points. Acetone's flash point is 4F I believe and extremely flammable; denatured alcohol around 50F. At room temperature both will give off sufficient vapor to support combustion. Use either in a well ventilated area and away from any ignition source.
Be careful.

Just use JB Weld... It will never move again guaranteed, the stuff holds engine blocks together. about 3.99 for the kit at any local walmart, auto store etc...