Is there any risk of collateral damage when doing a valve clearance
adjustment on an engine that is decades overdue?

Long version:

GF's parents are the original owner of a 1985 Land Cruiser, which they
graciously let us use when we visit.  Car lacks power and they don't
really take it on the highway anymore.  A quick google showed that
this engine has adjustable valves.  They don't remember ever having
the valves adjusted.  I also noticed the brake boost is noticeably
lower at idle, which I guess is a sign of weak vacuum from the intake
valves not closing fully.  Their local mechanic recently $oaked them
for new brakes because ZOMG SAFETY, didn't bother addressing a
destroyed sway bar end link which is visible right from the street,
and claims a valve adjustment needs special tools.  From a little
internet reading the "special tools" are a 14mm wrench, $6 feeler
gauge, and a flathead screwdriver.

Anyway, I'm interested in doing this work next time I'm there, but
since no good deed goes unpunished, and there is an obvious social
component here, is there any risk that doing the valve clearance on an
engine that's been so sorely neglected would cause stresses resulting
in other damage (broken valves, etc.)?

Thanks as always,

Alex