Youd have to cut/ notch the frame. the 20 is a twin where the 14 has a single cyl. the ones that came with twins have a slightly different frame.
If the price is right take it and take advantage of it being apart and redo it.... paint it up and use the GT14 as your "mule". Is this GT20 complete?
My GT19 was a smoker. I fixed it w/o a teardown. It needed a new breather. (the Magnums and KT series Kohler twins used the same part)
How complete is the GT20? Does the person who offered it to you have everything for it? Any extras come with it? (3 pt, tiller, blower etc?)
Another reason I'd grab it.... I think the GT20s were all power steering models.