You would have to remove shims in the 108 if any exist, not add them. You can run a 1 1/4 belt with the stock driven, but it also needs to be longer. Briggs cars back in the day had custom machined sheaves for the driven clutch. The outer one wrapped around the end cap widening it to accept a 1 3/8" belt. I do not know of any off the shelf clutch that will accept the 1 3/8" belt and fit the 7/8" shaft. The bigger problem is the larger diameter. If you find out the taper and offset of the engine, you might be able to get a 102C that fits.