You keep getting BMX freewheels because for years BMX was the main field that used single speed units. I think you want a Park FR-6 - and I think that will fit a lot of different freewheels. Your Pista probably came fitted with a fixed/free flip flop hub, and a standard threaded freewheel will fit on both sides just fine. You can fit just about anybody's standard freewheel on there, from cheapie ACS to slightly nicer Shimano to slightly nicer than that Sturmey-Archer to the expensive but supremely nice, durable and generally exquisite White Industries freewheels.

48x18 = 70-ish gear inches; you could also swap out your front chainring for a 42 and run 42x16 which would yield the same gear.