Can you weld the plastic radiator top?

Yes, It's very tricky and takes time to do it. Process called Thermo Welding. There is many DIY guide on the internet. basic 101 about thermo welding...

You have two options.

1st option is to buy professional plastic welder kit (starts $50-$300) that will melt plastic parts and fuse it back as one new solid part.
how does it work!
Simple you heat whole area where plastic welding will place, then once you reached required temperature for the plastic that you weld, use your plastic welding gun and the plastic ROD (ROD is new material that will be added to the welding process)
BTW very important you can only weld SAME type of plastic together, mixing it with other types of plastic may cause bad results.

Once you have both radiator and new ROD heated they will slowly join them self.

This is not easy but works,

2nd option: instead buying plastic welder you can try to use adjustable temperature or none adjustable soldering iron. On none adjustable soldering iron you have to play with the distance of the tip so you always have same temperatures on the part's it's very important to reach correct temperature for this to get successful results.

I hope this little info will help.

Boki