I do not have the book yet either, here is a suggestion though, perhaps you could ask the GM if for now you could just play the standard magus until you obtain the rules for the kensai? If not I imagine one thing they would give up is armor if it is anything like the 1st/2ed kensai, but I only learned 3.5 with pathfinder.
From what we have been told in the spoiler threads....
Kensai magi give up armor prof entirely and have arcane spell failure in any armor, the have diminished spellcasting (like the UM's cloistered cleric), they give up martial weapon prof. but get a prof. with any one martial or exotic weapon they want, at first level they gain weapon focus with selected weapon and gain the ability to add their INT to their AC, they lose spell recall (don't know about improved spell recall) and knowledge pool, don't know if it's at first level or later they treat their magus level -3 as fighter for fighter feats, at some point (again don't know exactly when) they are treated as full BAB for qualifying for critical feats, at 4th level (i think) they get an ability called perfect strike that 1)allows them to spend 1 arcane pool point in order to do max weapon damage, 2)allows them, on a sucessufull critical hit, to spend 2 arcane pool points in order to increase the critical multiplier by one.
That's pretty much what i remember, surely there are more in the archetype than those.
