can a pto be added to an international 350 utility

If it has a TA, it "probably" is set up for PTO, and which variety can "probably" be determined by the suffix on the serial number - "S" indicates gearing for IPTO, "R" indicates gearing for a transmission-driven PTO.

A "P" suffix is supposed to indicate IPTO gearing for tractors without a TA.

About the only way you're going to know for sure is to drain the transmission, take the cover plate off, and see if the gearing is actually present. Assuming it is, I "think" you can determine which variety you have by checking with the engine off: if the gearing turns freely with the clutch depressed, but doesn't when the pedal is out, it's transmission-driven; if it never turns freely regardless of the clutch position, it's geared for IPTO.

Good luck, and please post what you find out - this is still a vague area.

Parts tractor/salvage dealer is the only source I know about at this point.

The IPTO hooks into the driveline before the TA, so it should be independent (if all the parts are there) regardless of how well the TA is working. If you're having TA problems, first thing to do is to adjust the clutches exactly as the manual describes - you may get really lucky and find that's all that's wrong.