I have a few Tn28s, they are likely in the 75 to 85 psi range. Less than 70 usually causes issues with them. Main thing is that the cylinders compression is even.
This is the best link when working on a TN. There is an article at the end regarding compression.
Sea-Way Marine is an excellent source for parts for that old TN. Your model has the later universal style ignition, which was used on OMC twins, till 1974, so you can still find that stuff easy enough. Run that baby at 16:1 mix, too.
