"The J head is an older model of head that has step pulleys, to adjust the spindle speed, manually by moving the belt, it was a fairly reliable head, but with step pulleys you do not have infinate speed adjustment, as with a variable speed head."
Do like many did with the J head B-port. Not sure do you have a 3 phase power in your shop. If not, the get a VFD not only converts single phase to 3 phase but also give you infinite rpm control. As others said, converting older manual B-port to cnc cost more in the long run comparing to electronics repair/conversion of already cnc machine...
Forget about global warming...Visualize using your turn signal!