I adore Type 1 bodies. But the necks can be a bit on the fragile side, unfortunately. When a Type 1 neck breaks, you're better off just replacing the neck joint as it was fragile to begin with.
You could sculpt a new neck piece, but that could be equally fragile, and pretty hit or miss. It's best to create a new neck from something that already exists that will not break... but what? I generally make do with whatever's on hand.
A dowel rod seems like a good idea, but the neck hole is sort of rounded on the inside, so you'd have to cut into the pullip's neck to make it fit. I'd rather not risk cracking the neck with a drill, so I found an alternative in my tool box:
That is a screw anchor. You could use a screw as well, if you have a big enough one on hand. But a screw anchor fits my requirements of being sturdy, non-brittle, and making a tight fit inside the neck. This is the second time I've done this repair. The first time I used a different screw anchor and didn't need to do anything more than clip the tip off. The shape and thickness were perfect for a neck joint. But you're probably not going to get one perfect out of the box, so we'll modify this a bit.
I clipped the top and bottom until I had a good size. Then I put a little bit of super glue on the end that goes into the neck, and replaced the screws (except for the top one at the top of the neck). The screw anchor was contracted, but because of it's shape it still pushed out against the neck hollow for an extra tight fit, even without the superglue.
Since I clipped off the head of the screw anchor, I'll need a knob to keep the head from sliding off. Apoxie sculpt is made for just such doll repair, so I just sculpted a little rim where I wanted one and let it set overnight.
Now put the pullip's head back on. Better than new because the new neck will be more durable than the original. If your repair was nice and tight, the only way you'll be able to tell the difference is that missing top screw on the neck.