Help with grace notes!

Yes.

You will need to change the actual time signature to suit the bar (right click the bar and choose measure properties). Once you have entered everything select all the note you wish to be small and use the Note Properties dialogue to change them to small notes.

I don't think they are grace notes. Just smaller notes. You can enter them normally (I think they add up to 4 beats so no need to change the actual duration of the measure) and make them small (select them, right click -> note properties -> small)

This piece of music requires many tricks and cheats from MuseScore in order to render it as written. Finding those tricks and using them is all part of the fun but, for starters, here is my attempt at it and I have changed the measure duration, made some notes small, changed the beaming of notes, changed the angle of the beams in places, done some cross-stave beaming, made some rests invisible and added others (the measures don't quite add up exactly) and also used the Z palette to add a couple of sharps after the bass clef, etc.