The ST170 does NOT have tappets, it has solid lifters so it is generally a bit louder than an engine that has hydraulic lifters. When the lifter buckets wear additional noise will be heard as the lifter buckets will need re shimming. But 80'000 is not enough milage to require this really. Remover the cover over the coil and try and peer down to the inlet manifold, you will see a cable running to it attached to a lever. Get some one to turn on the engine you should see the cable move as soon as the igniton or engine starts, if this does not happen the IMRC is fookbar and needs replacing, or the lever is broken on the inlet manfold. A broken lever does not tap tap tap but a mashed up IMRC box does as its the gears inside you can hear.
IMRC is about £180 for the part alone.