AmyK, unless you're into tinkering with things changing the handwheel may not be a straightforward project. You will not find a larger replacement handwheel, what you have to get is a large pulley with the same shaft diameter as the machine and then ensure it can be locked to the shaft. I have had to drill and tap more than one pulley to lock it to a shaft.
If you're not keen on messing around with the gradient strips in your servo controller I think your best bet will be to sell the servo and buy a new one (I'm not aware of any newer ones that use the optical gradient control like yours). Even if you buy one of the inexpensive generic servos on ebay it should work better than the one you've got, and if you fit a box speed reducer you'll be able to make the needle crawl. If you can handle a screwdriver and spanner you should be able to fit the reducer, or get a friend with a little mechanical ability to help.