Most breakdowns are either electrical or fuel related. Examine your system and work out the what iffs...
Fouled spark plug- set of plugs and a spanner.
Blown fuse- couple of spare fuses, tester or meter, torch and a screwdriver.
Broken wire- wire strippers and a screwdriver or pair pliers. Spare wire.
Blocked filter- Screwdriver to remove hose clips, maybe a spare filter.
contaminated fuel- youre a bit buggered there. VHF?
Split hose- screwdriver and knife. Spare hose clips.
Points awarded for the most bizarre.