The engine you have is perfectly fine (except for age and wear) and you should think about a rebuild whether you rebuild the one in there now or buy a spare one to rebuild. If money isn't an issue, your only two options as upgrades would be the B18C1 or B18C5. Keep the engine you have, rebuild sometime in the future, and then decide to mod it. Don't mod an engine that you know will go to crap at any minute. You will be hurting financially when all your money went to mods and your engine blew. You will be without a car and those mods you paid so much for are now just pieces of metal.