believe the 3813 radiator will work fine, as the tractor frames are the same, and there's at least one, i believe up for sale on egay - i just bought one last week
i believe a new rad will run about $260+ shipping from boats.net and they're about the best prices on the net for honda parts - here's a link to the selection of lawn tractor parts catalogs -http://www.boats.net/parts/search/Honda/Lawn%20Tractor/0/parts.html - you'll have to determine which 4518 you have
believe you'll want to get a PTO off egay - prices on those are fairly steep from honda, even at boats.net 20% discounting but the parts schematics at boats.net will help you know what you need
the surplus center radiator has a copper core, like the honda's, which is nice as copper transfer heat faster than alum - but has the bottom hose connection facing down which is odd - you could have a radiator shop unsolder that nipple and turn it up - the honda radiator has both connections at each end of the top - which is odd in it's own right as most radiators have one at top and one down low -
I still have a 4518 cyl head, complete w/cam, valves, governor, etc available, if anyone's looking for one
thanks for the tips on finding parts. I will give it a few weeks and if I do not find anything I will consider parting out the machine. It has a good deck looks like it was down for a pto clutch and the PO did not want to spend the mony to fix it. I got it from the shop and they could not find the parts.