I'm fairly certain the only difference between the 1.5 and 2 hp are the intakes and head gasket size. 1.5 had thicker head gasket, 12 mm intake; 2 hp had thinner head gasket, 14 mm intake.
There are plenty of performance options that you can try that won't change the stock look (drill out carb/intake; port cylinder; remove head gasket for more compression). Search the forums. (edited)