A shul has a Torah that is too heavy & isn’t being used. Are we allowed to cut an inch or two off the top & bottom gilyonos to make it lighter?
Yes, this is permitted. The parts that are cut off should be placed in genizah.
Many sources discuss the halachic status of pieces of kelaf that were cut from a Sefer Torah (concerning the question of what these pieces of kelaf can be used for; see for instance Darchei Moshe, Yoreh De’ah 290; Rivash 32; Semag, Lavin 3; Masas Binyamin 100).
From the extensive discussion in rishonim and acharonim it is clear that there is no prohibition of cutting the kelaf away, and only of using the kelaf (according to the majority opinion) for purposes that are beneath the Kedusha of a Sefer Torah.
If the Sefer is not being used because of its weight, it is the glory and honor of the Sefer Torah that part of the gilyon should be cut off, so that the weight should be reduced and the Sefer used.