Well, replacing golurk is very difficult imo, i dont think there's any similar pokemon in the tier, or even in the whole game (yeah, there are other subpunchers but still)
Mantine is probably the best switch for gastrodon i think. Cradily is an option, but it makes you weaker to fighting and bug attacks, which is not good.
For replacing golurk, you have to make the choice of choosing another subpuncher or just going for a different thing. Iron fist hitmonchan can fit golurk's role OK, and so it does Hariyama. The matter with doing that is that then ghost types would make fun out of you using 2 fighting types, so then you can switch Scarf Krookodile for medicham. It could maybe not be as strong, but with Moxie, it does the work out while mainteining pressure. I know honchkrow can handle ghost type well, but things like RestCM Cofagrigus just laughs at honchkrows attacks except of sucker punch, which cofagrigus can just pp stall and recover the damage with rest.