Welcome to the forums.
Looks like quite a few coolers will be a tight fit with those heatpipes, been doing some research and it seems the space in between is approx. 130mm x 107mm, so that's the restriction on size for the cooler.
The stock coolers with the pushpins can be tricky to fit actually, the design they've used is very awkward IMHO and I have struggled before now. What I'm trying to say is, if the supplied cooler fits, you may as well try again to fit that, because all the cheaper coolers out there use the same mounting method so they'll be just as difficult/awkward to install.