1963 Evinrude 28hp speeditwin. Wont stay in forward gear

Hi everyone, having more problems with my speeditwin. It currently does not like going into forward gear (I’m considering forward as being able to move the throttle all the way to fast). I can hold it forward and it will go all the way to fast. I did beat the shifter arm on the engine with a rubber mallet and then it temporarily fixed itself. It would shift easily into all the gears and click into each one but now it’s back to how it was. I’m thinking it might just be in need of oil throughout the whole mechanism because the throttle is hard to move as well.

You are turning the prop when shifting I hope.

....oops