how to repair engine if I can't get manual

Hello everyone!
I have to repair the engine of some antigules agricultural tractors that I have and I have searched and I do not find the manuals as a part.
One is a Fiat 655c and the others is a fiat OM 50C, both are caterpillars. I thought that perhaps putting a sign of reference to a butt screw for example, loosening them a little and then tightening it again until putting it in their sign with the dynamometric key and thus be able to find out its squeeze, the same I am thinking about a barbarity, or if anyone knows any trick to repair. Antiguos of what is very difficult to find workshop manual. Good thank you very much to everyone!

A good measure to adjust the cylinder head bolts when the torque is unknown is to apply 1kg/m per millimeter in the bulon diameter.
This technique does not apply personally but an old mechanic told me that they used it years ago to adjust the career cylinder covers.

The torque can be calculated by the length of the bolt the thickness and the material there is a table on the Internet, but it is not so complicated to deduce it by logic usually in normal displacement engines the tightening round between the 70 pounds and the 120 pounds.

Thank you so much!
Would this be apicable to connecting rod stages and the banks of the rocker axis?

If you are a friend to all kinds of bolts, it will depend on those three factors the maximum tightening they will have before cutting, look for the boards, put in Google table of tightening pairs, or table of torques for bolts and something will come out.