I would take it back to the same guy who had to replace the seal. How many miles since he did the work? Sounds as if he may have done something to accelerate this issue.

Did you do anything to precipitate the knocking? a high speed manual downshift? Anything?

A wrist pin is usually noticible under load. What may work is removing plug wires one at a time to see if the symptom ceases on the suspected cylinder.

As far as any upgrades, you have an original engine correct? I wouldn't go hog wild as if you grenade your engine and it's unrepairable, you will lose alot of value. When was the engine rebuilt last? Do you know if you are running stock compression? Cam?

I would start with why it's broke and go from there. A good set of forged pistons with valve reliefs for 72 cc heads and a 9.5 to 1 CR is a good start.