I started working on two units of jetbikes today.
The first hint I will share with you is that if you are wanting to build jet bikes, don't buy them individually. They cost 15.00 each. Instead buy the box of (3) Shining spears. They come with three full jetbike sprues, and a box of pewter upgrade bits. The box costs 40.00 so it is a bargain for the three normal bikes and you gain some stuff for your bits box.
Secondly, converting one of the bikes over to a Shuriken Cannon is pretty painless. To start I took my razor saw and cut the fronts off the pair of shuriken catapults. I took a Shuriken Cannon leftover from either a Wraithlord or Wave Serpent and cut it just below the barrel. A quick drilling and pinning later and you can see the results below.