I have heard of guys doing this in the past with good results but os power has some good advice there. Compare the port heights of your P5 sellve with the port heights of the 421 sleeve (from the bottom edge of the squish band at the top of the sleeve) before commiting. The Novarossi line have been changing little by little and the newer stuff might not be the same.
"I have no idea what you're talking about, so here's a bunny with a pancake on it's head."
In mind and body, we're all heading towards old age. It is slower to go by bike.