If the track is still in good condition you may be able to use a heat source like a hairdryer or heat gun to warm it and close it up. If it has worn then it needs replacing. You can consider using aluminium mast track from the 29er for the first 40 cm at the bottom of the mast which can be closed up with a vice or mole grips. TIP: To make it easier to raise/lower the sail and put less stress on the track, do so with the gennaker slack and the forestay loosely tied at the bow - this allows the top of the mast to flex more easily. Once the main is up, tighten the gennaker then the forestay as pulling on the gennaker pulls the mast tip forward.
This may also help http://wetaforum.com/forums/topic/mast-track-repairs/
If the track is still in good condition you may be able to use a heat source like a hairdryer or heat gun to warm it and close it up. If it has worn then it needs replacing. You can consider using aluminium mast track from the 29er for the first 40 cm at the bottom of the mast which can be closed up with a vice or mole grips. TIP: To make it easier to raise/lower the sail and put less stress on the track, do so with the gennaker slack and the forestay loosely tied at the bow - this allows the top of the mast to flex more easily. Once the main is up, tighten the gennaker then the forestay as pulling on the gennaker pulls the mast tip forward.