It looks like the stock chain on the 2007 klx250s is EV520LV-O, 106 links.

Does anyone know of a master link that will work with this chain? Will other EV master links work?

I don't see any info on the LV chains on EV's website.

 

From my limited experience so far, each chain "model" (even from the same manufacturer) has a unique master link.
I'd be quite happy to be wrong on this.

 

You're pretty much right, but you also find that many brands of similarly designed chain will allow interchange of master links. Of course, this assumes we're talking only about the exact same size chain...530, 520, etc. All chains in the same overall size won't have the exact same size plate and/or roller dimensions in areas like plate width, pin size, o-ring dimensions, etc. However, you can usually find a master link that may be practically an exact match to your chain. Still, it is highly recommended that you buy/order an exact matching brand, model, and size mater link to match a given chain.