Food for thought:
Was reading through another thread where the OP had shown installation of a new clutch pressure plate with existing bolts. Someone had commented, hinting maybe that wasn't a good idea...and others had chimed in saying it should be fine. The uncertainty took me to Bentley, which says to clean and dry all six pressure plate assembly mounting bolts and all six holes in the crankshaft flange.
It goes on to highlight the need to coat the bolt threads with Loctite, install and then torque. The torque value noted for bolts with shoulders is 72 ft-lbs.
The bolts are M10 (0.394 in)
The grade is 12.9
Proof strength - 140.7 ksi
Min. yield strength - 159.5 ksi
Min. tensile strength - 176.9 ksi
The Machinery Handbook notes the recommended preload (Fi) can be determined from: Fi = 0.75 x At x Sp for reusable connections, and Fi = 0.9 x At x Sp for permanent connections. In these formulas, Fi is the bolt preload, At is the tensile stress area of the bolt, and Sp is the proof strength of the bolt.
It also notes that if measuring bolt elongation is not possible, the torque necessary to tighten the bolt must be estimated. And if the recommended preload is known, use the following general relation for the torque: T = K x Fi x d, where T is the wrench torque, K is a constant that depends on the bolt material and size, Fi is the preload, and d is the nominal bolt diameter.
Because K depends on the bolt diameter and size (and finish) it's kind of the wildcard. Best suggestion from the Machinery Handbook is to use K = 0.3 for non-plated black finish bolts.
So if we assume these bolts are reusable then Fi = 0.75 x 0.0899 in^2 x 140.7 ksi = 9.49 kips
and T = 0.3 x 9.49 kips x 1000 lbs/kip x 0.394 in x ft/12 in = 93 ft-lbs
With Bentley recommending 72 ft-lbs it seems this case falls within the 'reusable' criteria.
However, if you (or the prior installer) were to use a plated bolts or lubricated bolts, K could easily drop to 0.18 and the corresponding wrench torque would decrease accordingly to 56 ft-lbs. Introducing 72 ft-lbs (recommended torque) in this instance would achieve a preload that's 28% more than intended, which would take you out of the 'reusable' criteria.
My takeaway on this connection...if you don't know the history then play it safe and replace the bolts.