Try asking at the forum on AMIEN.org - http://www.amien.org/
Wear a flameproof suit and avoid mentioning 'museum' and 'money not available.'
The old/yellowed paint might be varnish. You can hope that the mural was varnished with an art-friendly varnish and that nobody since 1920 has covered it with conventional varnishes.
Art-friendly: removable with alcohol or (genuine) turpentine... only be sure the latter does not remove pigment. Try alcohol first -- shellac thinner grade ethanol, not isopropanol.
If the canvas has been uniformly adhered to the wall, you might have no cracking to speak of, assuming the wall itself is still sound and not cracked.
Some public-commissioned murals of this era are true art treasures. Please, research the artist and the commission as deeply as possible. It probably made the local papers when it was unveiled. If this is a treasure, I'd leave it alone and urge the museum to do fund-raising for a professional conservator.
Being terminally cheap myself, I'd cozy up to any and all art museums in Houston and see if there's anyone friendly enough to give you free advice. I've gotten incredibly good information from contacts at the Boston Museum of Fine Arts, even though they (the conservators and curators) are underfunded and time-constrained.
As always in a highly-technical field, you may be rebuffed sharply by those who suffer no fools, but just set them aside and look for more human sources.
Judging a Manet from the point of view of Bouguereau, the Manet has not been finished. Judging a Bouguereau from the point of view of Manet, the Bouguereau has not been begun.
--Robert Henri, The Art Spirit