Erratic HP 9100B

[Cosmo]

Dear All, I got an old 9100B that works erratically. To be precise it lights up but the CRT disappears after keying some specific keys, mostly advanced scientific functions such as the trig, log, exp. The remaining basic functions (add, multiply, square root and so on) work well including storage in the various registers. Opening the calculator I saw that the gummy wheel was entirely disintegrated and the gum fell on the mainboard (gummy spots). What would be the best way to remove those gummy spots (which chemical are safe) ? Thanks in advance for your kind help regarding such a wonderful machine.