How to clean vintage watch with no WR?

Hi all!

Today I received this beauty - Seiko C515-5009 from 1978.

It is used, and at some places it screams for cleaning, but there is an issue:

Originally it have 3ATM water resistance, but I already opened the back case and noticed, that side pusher buttons definitely will not hold any water from getting in (one is missing the gasket completely, other three do not seems to be in good shape too).

With bracelet it is easy - just remove and I can do that thing with soup water and toothbrush, any suggestions for the case?