Replacing chain ring

165mm is for sure the crank length. 144 bolt circle diameter is the old Campag road standard and I believe is current on their and many other track chainsets. It will take a ring down to 42t although 48x18 is the perfect riding around fixed gear.
Track bikes often have a 1/8th inch chain and ring/sprocket (slightly wider) but a "road" 3/32" chainring will work OK. TA make good rings in quality alloy.
If you go from 52t to 48t you may well have to take a couple of links out of your chain.

Thanks, will a 48x18 be able to tackle hills? I know it mostly leg muscle/power but will that ratio be enough?

Also, I was looking at this vhttp://www.evanscycles.com/products/token/track-chainring-1-1-8-ec020838?query=Chain%20ring but it doesn't say the BCD... It does however, as you mention, have 1/8'' but thats nothing to do with BCD right?