Could you build a trellis on two sides of it, and add morning glories?

please show a picture of it. Include surroundings.

First - what are the required clearances around that boiler according to your local building and fire codes, and the manufacturer's specifications?

Second ... please show pictures of the boiler from the places you want to screen.

You don't mention your zone, but evergreen star jasmine grows fast, would take over a trellis in no time and blooms in spring with heavenly scented small flowers. You could plants shrubs or flowers at the base, perhaps a couple of small trees for interest.