Life expectancy of a 400?

I have a 79 KZ400 LTD with low mileage for age, near mint condition (few scratches on tank and light oxidation on cooling fins), and mostly original other than general wear items (rubber, chain, rear shocks, coil, plugs + boots, etc), but it's smoking pretty heavily out of the right pipe. It's always smoked after winter but never this bad. Not sure if it's from age / wear or just because it's been in storage for longer than normal. The bike is sitting right around 13k, but is also going on 43 years old and other than the smoking has no issues. Wondering if it's getting time to do the top end soon?

Sounds like valve stem seals.  Rubber parts age.  Rings may be gummed up somewhat too.  An easy update if you are capable of doing the wrenching.  Otherwise it will cost more than the bike is worth.