I am looking at running a high pressure pneumatic system using off the shelf hydraulic cylinders, which as I understand can be used as long as they are down rated by 50%

For off the shelf hydraulic parts, can these be modified, and to what extent, and does this required re-certification?
For example, boring out ports for faster flow rates.

If I were to manufacture my own cylinder, the rules state that all custom components 'must be pressure tested and certified to 120% of working pressure.'
Are there many places that will do this, and what are the typical costs involved?