Engine swap is easy way rather then overhauling the old engine.provided u get a gud engine to swap with.what is ur current engine ?? 1.3 or 1.5.
As for the new engine,it all depends on you.I assume all D series engine will comfortably fit,U can start from D13B4 (came in city 00-03 model) and go upto H22 or H24 even.
But the most popular swap is D15B Vtec , it's economical yet powerful engine and runs gud on CNG.
However,u'll be converting from Carby engine to EFI which means wiring has to be changed as well.