yes i was thinking 100 or 110 myself....... are these big four strokes tempermental or are they easy to tune?

i have an o.s. .52 fs surpass on my wifes lt 40 . it took a little tinkering to get it just right but i have not touched the mixture needle in months and we fly a couple times a week.

would a used engine be okay or do youall suggest i get a brand new one?