You can buy tubular rivets that will be an exact replacement for the rivets you're removing. You'd also need the proper tools to pein the rivets to hold so for a one time deal it could get pricey. You have to be careful using pop rivets on motor covers. Often times the clearance between the outer cover and the internals of the motor are very tight. My personal opinion is that pop rivets look horrible as replacements but if its for a user piece and you're not shooting for a "restoration" I'd say go for it if you've got the clearance.