I have a 1999 civic si and i was wondering would i be better off fully building a B16a2 or getting a swap?

Hi, It depends on if you want to run naturally aspirated (NA) or forced induction (FI). If you want to run NA then I would go with the B18C5 for the higher compression. If you want to run FI such as a turbo or supercharger then I would suggest the slightly lower compression B18A1, or your B16A2 would also make a decent starting platform. Budget build = B16A2, Money is no object = B18A1 or B18C5. Hope I helped.