Another Cresta owner could answer if they ever had any problems with cantilever setup / compatibility & fit after a changeover in wheelsize, or if their frame failed at any particular point, things to try & look out for.
In my case, I'd go with a change of bars and to a more upright riding position and wider tires. The widest available for the 27" on this bike is a 1-3/8" apparently. Whether fenders would fit afterwards is unknown and fenders would be nice.
The Cresta is an elegant looking bike, like so many similar of the era. It is fairly light (MUCH lighter than what I'm used to). The roads in my City are drek and wider tires are a must. Ultimately, despite the Cresta being in wonderful condition, my size and a steel frame, I would happily trade for a top quality steel hybrid from the 90's or a more modern one both of which could take large volume tires. My hands have taken a beating riding over the years and I have palsy in one as a result. Also, swapping parts out on a bike quickly adds up in costs and I'd rather just put that into purchase of a complete bike. BUT, in the meantime, I may just ride the Cresta for a while (with scrap parts swapped out for the bars/shifters) with some 1-3/8" tires on the original wheels.