Steve,
Early Massey Harris Type 1 open crank hit and miss engines were made in 8 sizes,
1.5, 3, 4.5, 6, 8, 12, 15 and 20 HP
Type 2 engines of both open crank and later enclosed crank throttle governed engines were made in 4 sizes.
1.5, 3, 4.5 and 6HP
Hope you can post us a photo.
Malcolm.