Build a new one and use your old one to put all the stuff that ends up in the shop that shouldn't be in the shop.   Old one probably doesn't have roof overhangs.  Overhanging roof with soffet and facia is important for ventilation when properly insulating a morton with all the attic space in between the rafters.  Is it a sissor type truss ?- that makes it a little harder to line the ceiling by the big door.   Is there clearance for overhead doors between the header and trusses?- In mine there is not.

A few years ago when I was being stingy I started to convert a 26x40' barn into a shop.  I got about a third done and was already unhappy with the effort it was takeing to make a shop that I could see was already obsolete.
I shifted focus to doing the same thing to my 54x72x14 morton but eventually  I just decided to pave it and am making plans to build an insulated and heated shop from scratch so I'll have twice the space and can fit a truck with a 53' trailer in it.  I am already glad I didn't put any more effort into the old barn or morton and put my efforts to making $ instead.