Every small engine I have seen has a governor speed control that
automtically adjustd the throtlle to try to maintina a fixed speed. If
the engine also has a user throttle control, this just adjusts the auto
setting. So yes you should be able to add a spring or something to
slow down an engine.

CAUTION, an engine with a defective governer can run VERY fast and self
destruct prehaps in a dangerous manner. Do NOT do anything that
overides the governor control such that it cannot slow it down.

Mark