It is my understanding that an unaltered semi auto receiver that is converted with an unregistered sear is a "collection of parts". As such, all the parts that together make the firearm shoot more than one round per pull of the trigger, a machinegun. Could the trip lever be replaced? Who would know if the trip lever was original? But if it could be proved that the trip lever wasn't original, how would the FTB view that?
Could you replace the grip housing? Again, I don't know. Several manufacturers that did such conversions put their information on the steel grip housing instead of the receiver or barrel. All required markings on Title II firearms when they were originally registered must remain. Only the original serial # must remain on Title I firearms, but all original required markings must remain. So how would the FTB view that? With a market value over $25,000, I wouldn't want to risk that. YMMV.