Perfectly safe to fly with only small cracks in the wing spars
if your really lucky some of the concord engineers might post pics of the cracks in concord's structural frame which it operated with perfectly safely for many years. I don't know if you would class as small though something you could get your finger into.
Condition monitoring of cracks in engneering terms started back when we started learning about fatigue and welding boats in the second world war. You might be alarmed to know how many cracks there are in your local nuclear reactor pressure vessel in various loading lugs and other structural items.
opp's but forgot the orginal posters question.
The minmum equipment list for a GA light aircraft is minimal apart from the bleeding obvious about wings being on engine working etc, clock, wet compass, ASI and altimeter. The rest if operated on a AOC will have a time to get fixed and restrictions on what the aircraft may do etc eg DAY VFR only. Now in theory your plane owner should US the defective Instruments, an engineer will isolate them and placard them as US.
The structural issues I would hope that this is being run in tandem with an engineering organisation. Each aircraft has a structural rectification manual which pilots never get to see. It has known age related structral fixes and also accident fixes, it details what to check and also whats the approved method of fixing the problem. For a crack it might state for example
1. Crack under 25mm long and no wider than xxx slip guage then montior.
2. Crack over 25mm under 50mm no wider than xxx slip stop drill. If already stop drilled crack must have extended by 10mm before another stop drill.
3. Crack over 50mm long or wider than xxx slip refer to xxx-yyyy-zzzz repair plan.
My company aircraft currently has 2 cracks in the first example. Every time the engineer gets his hands on the aircraft for a check he pokes at them with a slip gauge measures them and releases the aircraft to service. When the crack gets longer than it says in his manual he will drill it. But the crack will be getting removed anyway on the next C check because the main cost of dealing with it is the man hours stripping the wing down to remove it. C check everything is in pieces anyway so the cost of the job goes from being $15,000 down to $900 for part only.
If you are concerned that the owner isn't playing the game just use CHIRP to highlight it. If you start dealing with it yourself it will just end in tears because
A) most pilots don't have a clue about what engineers can and can't run with.
B) It may very well be all legal.
C) Nobody will know its you thats made the fuss.
So by chirping it
A) the plane gets fixed
B) It stay's as it is
C) Some dodgy engineering outfit gets multiple inspections for releasing defective aircraft and the country is a safer place.