Worm gear replacement is major overhaul. To change it requires EXTENSIVE disassembly of the mechanism (once it's removed from the projector of course!). Once you start dissembling the mechanism you lose all the critical adjustments which require the special (expensive) Bell & Howell tools & jigs to readjust/realign. The new gears are colored black to distinguish them from the old style plastic

It takes at least 4 to 6 hours to open / replace that worm gear / and to REASSEMBLE your B&H correctly.

Average cost worm gear replacement job costs around $300 per machine plus parts.

One could replace only the cracked worm gear, but it is almost the farthest thing into the projector. Half the labor is expended just accessing it. And, the projector probably wouldn't be happy and might have wow in sound and picture instability, because sprocket spindles may be dry of lubrication, a sprocket gear may be warped, sound head needs attention, shuttle needs adjustment, uneven take up drive, etc., etc.