Lovely cruiser. My advice will be stick with the existing engine as you side it is in great condition.
If going for swap then 1 HD-FT (Multivalve) will be the best option but as i hear its a bit problematic sorting out the wiring but totally possible cost involved will be around 3 lacks and i think your existing engine should go around 2(that if you keep it and sell it to private buyer, in market i dont think they will give you good price)
Now if i recall correctly the 1 HD-FTE(Multivalve with EFi is 100 series and engine It can be fitted as well but its not a Bot ON swap you will have to move the engine a bit back and if you use the existing gear you will have to make a plate by which gear and engine will be mated. its a very complex process, and cost will be 5 lacks or more.
Though Idea of a 1uz 60 series sound fun too