If you can't find a shop to do the work & you decide you want to try and fix it yourself go buy a couple of gallons of vinegar.
Calcium(which is most likely your build up) hates Vinegar, remove the radiator & lay it flat & fill it up & let it sit for a few days. The vinegar will eat the calcium, this is what I used to clean my C-pap machine and other appliances that hold water & get a calcium build up......joe