Willit
Silicon is a good insulator so is no good for the actual coil. There was a silicon jacket with a moulded in groove on the inside for water to flow round but I haven't seen them in years (they leaked like mad anyway).
Get your copper tube from your friendly car brake fitters (I've got it from kwikfit and similar), just ask them for a couple of feet of the tube they make the brake pipes from. (Ask for half a metre if they look younger than 30 ;-). My last lot was 10 metres from a car accessory shop for £5 - should last ages).
Find something slightly smaller diameter than your motor (piece of tube, brush handle or whatever) and form your loops around this. The loops will not be very tight round the former, but that's ok as they can be eased open to go over your motor, then they'll be tight. You can also use plastic cable ties round the coils to keep them in place as the copper (with water flowing through it) will be cold.
Cheers
Danny