My bike's tappet noise becomes much high after riding it for 4-5 kilometres so one of my mechanic told me to install adjustable pushrod which basically comes with royal enfield old models. My question is what should be done. Can my mechanic's suggestion solve tappet noise issue?
Tappet noise is a "feature" provided in RE Classic 350. It doesn't do any harm as such, except for annoying the rider/owner of the bike.
To answer your question, Yes, it is possible to swap the tappet rods. RE recalled Himalayan few months ago to swap these tappet rods. Check with your service center to see if you can get the same tappet rods installed. Basically, they're longer than your usual tappet rods and they have a groove on them.