He wants longer stem, but missions have 1.5 head tubes, headsets, steerers and stems instead of the normal 1 1/8th, seeing as there are no trials stems made in 1.5 (as far as i know), he needs to buy a new headset and forks. So yeah, he is looking for a new headset, but he needs a 1.5 reducer headset so he can run a normal 1 1/8th fork and stem
This should do the job http://www.chainreactioncycles.com/Models.aspx?ModelID=25722 With that headset you can use any 1 1/8th trials fork you want: http://www.tartybikes.co.uk/category.php?id=13 and also which ever stem you feel will suit your riding best: http://www.tartybikes.co.uk/category.php?id=29
Hope that was what you were looking for
EDIT: Although according to the mission website they have 1 1/8th headsets http://www.missioncycles.co.uk/Folding_Tri...1KEY%3A30PAR%3A
dosen't look 1 1/8th to me though :S