I would suggest a heavy duty cleaner such as Muriatic or Oxalic acid. Just be aware that either acid is powerful enough that it requires neutralization after use, however most times it won't unplug a radiator too far gone because it can't flow in there to get at all the blockage.
That's not saying don't try, but it is likely too far gone at this point.
Also; to make a suggestion, if not already equipped, the forklift should be equipped with a coolant filter once this is all done with and the radiator is once again operational.
There are a number of different ways one can go about this.
"Real" coolant filter heads are silly expensive, as they have a special thread. (11/16-16) However, you can use a marine fuel/water separator head, which has the same thread and use a Wix 24069 or equivalent. Donaldson/NAPA 4070 is another good choice.
I personally use a Summit SUM-G4985 oil filter relocation kit along with a standard Toyota oil filter on my Supra because that's what I had, and it's easy to service.
"Real" coolant filters are inherently better, but something is always better than nothing.
After one year of service, I opened the filter and was impressed with the amount of junk it picked up.
Total cost invested for me was about $40 including hose and fittings. Plenty worth it.
I run it inline with my throttle body coolant hoses, this setup is to be used as a bypass filter, mine is plumbed with -6AN hoses.
::edit:: also, make sure to get a filter without coolant additives built in.
you need to come south, lot's of rad shops here in LA. 
