Make a call to MSD tech support, they could walk you through the procedure and recommend a part number for the correct gear. I've never had an MSD unit, but if they're designed like the Ford unit, there's a small roll pin that's driven through the gear and dist shaft. The pin is driven out with a small drift which allows the gear to slide off the shaft. There are a few different diameter shaft/gear combos on the Ford, and there is usually a thrust washer(s) between the gear and the housing.
