1980 yz125 wont go into first or neutral

Photo?

Not that it has anything to do with the shifting issue, I personally would not let a new top end to idle for 30 minutes. For good ring seal, you should run the engine up and down the rpm range to get a good seat in for the rings. Riding is fine in my opinion, just don't lug it or scream it. Putting a load on it through a good portion of the rpm range is what you want.

As to the shifting issue, could be something in the shift drum, the shift arm, a spring on the shift selector fatigued/broke etc. Only one way to know for sure is to pull the cover and clutch off to expose the mechanism. Parts actually aren't that hard to come by.

Good luck!

Yes photo of the bike in general

Check out those bars!

A couple of parts you will need to replace,top pic # 2 shift segment. Or it could be loose but the pins wear.

and #1 shift shaft. These are the two parts that wear out on the G model so replace them and you will be good to go.

And another thing, dont let a bike idle for 30 mins thinking your are breaking it in.

Shift springs #8 in both photos or guide pin #6 in the second photo are your first go to places to find the problem and do not require a case split

#6 screw loose
this was a issue , service bulletin on this
to apply loctite