If you cannot take care of a vanilla GA15DE, please don't venture into anything complicated than that. The SR engines are, for the most part, performance oriented engines, if you do not know how to work on your own car, do not go for engines like these.
FYI, all "used" engines are "slightly weaker" than when new. And if that sound was from the conrod, you wouldn't be driving that thing. It's probably from worn big end bearings in the crank, caused by low oil pressure. get your oil checked.