New OEM valve springs are probably adequate for any sane RPM. Over-revving a glorified tractor engine makes about as much sense as putting racing stripes on a snail. The cams that most people without a clue put into these old engines don't make good nonsense.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!