these things reach peak hp at 4800 rpm at which point the early has about 105 ft/lbs while peak torque is achieved at 2800 rpm and is about 130 ft/lbs, i believe that the engine was intended to be used in this range(redline is 5400 and normally from my limited experience there is about 500 rpm of overrev designed into most engines), i tach it to 4400 every day and it handles that perfectly fine.
Other than the historical head and head gasket issues, and the timing chain guides, these engines seemed to be designed/built to go 250,000 to 300,000 miles without a major block rebuild.
Driving conservatively isn't going to hurt them and technically does reduce long term wear factors.
I think following the recommended replace/repair and preventative maintenance is key to extending the life on these early Toy engines.
In my 22R and my 22RE I don't recall ever floating valves even at 6,000+ RPMs, so taching it up once in awhile didn't have any adverse results.
Obviously, overheating them will almost always cause a head failure.
Gnarls.