Briggs and stratton 3.5hp and 4hp interchangable parts?

Hello I have 2 mowers a 1995 briggs and stratton 3.5hp classic and 4hp quantum. The 4hp keeps fouling the carb now it starts and dies and smokes out the exhaust or carb on shutdown when it ran. The 3.5hp wont start and backfires every 3-5 pulls my question is can I interchange parts like the valves to make the 3.5hp run again? might sound silly but the hp is similair any help to get one or both working would be much appreciated. Thank you