Of course, shop manuals are the best source of information for torques of donor components. But there are many fabricated parts with standard fasteners. For these, there are pages on the internets listing normal torque settings for various grades and sizes of bolts. E.g. 75% of proof load for 8.8 grade M12 is 88 Nm...
http://www.cncexpo.com/MetricBoltTorqueNm.aspx
So, as a rule of thumb, ~90ish Nm for M12, ~40ish for M10, ~25Nm for M8, ~10Nm for M6.