If it is a Honda... Then almost any Honda dealer should have parts for it.
Call them on the phone and ask them.
Then go there, and look on the microfiche machine (fish) and look for the exploded view of the carb, so that you and they both are talking about the same part.
Then they might have to order it (you pay first).
In my opinion almost any oil car or bike 5w30 or 10w40 or 20w50 oil is fine.
Especially in a low HP motor like that.
But in general you should use the lower number in cold weather, and the higher number in hot.
So 5w30 is fine.
Maybe change it in June to 10 or 20w.
There is one good thing about Honda's... they are usually very well engineered...(even the stuff 35 years old) .. there are very few bad things about Honda's in general.