1 - G1 Mirage missing only the gun - is it ok to replace with the KO gun and never really look again?

2 - G1 Metroplex missing both fists - could i use fists off the KO metroplex?

3 - G1 Prime missing some rubber wheels - could i replace them with some off a KO version?
Yes to all long as it's not horribly noticable. Some of the genuine parts on the Hasbro reissues are so stupid, large and without chrome I think they are worse than KO parts and I've started replacing them with older genuine playworn parts just coz they look better and I would be willing to use KO parts if they are more G1 accurate than the reissues.