The subaru 35cc has 1.6 hp,go with tanaka40cc (2.2hp) if you don't mind a louder motor,or check out the mitsubishi tle43,(2.3hp)aquiet smooth running motor, the gp460 (4.2hp)is really fast, but noise could be an issue. Check out the gebe-460 thread. If you do go for more power, you'll go through belts more frequently. I'm not aware of any modifications available for the robin subaru.