Well, I am still laboring on this piece too since months. Same situation in my case as for Pianolady, I need to memorize the whole thing, but it is worth the labor - in order to play it on suitable occasions from heart anytime.
The Presto part: I do practise it from time to time again hands separated (also from memory hands separated). You could give it a try to raise the speed hands separated first. Now I am trying with metronome, put on half notes (I am just on about 100 BPM half notes, not really presto, but let's see ...). I find the groove tricky, but not so much the speed. I think that the Presto part is not that difficult, because the notes are "ergonomic" to play.
Are there special bars what makes trouble? Which bars? We could share our fingerings, if you like!
In my case I have trouble with the last 2 runs with both hands. Especially the last run, played in a decime distant. Don't get those runs up to speed right now, perhaps some months later