Fuel Injection pump might be the easiest and cheapest of the two to fix, but whenever buying something that does not run, you often are at the mercy of the honesty of the seller as to what else is wrong, sometimes even when it is running. Injection pump rebuild around here for my '58 Ford 861D I'm told can run about $5-700. I'm not real familiar with the Fordsons though or your local pricing. About $2k is the most I'd want to pay for one that (claims it only needs a fuel injector pump) does not run and gamble another $1k to get it working. Unless you could get it for little over scrape metal price, you may end up paying less in the long run for a tractor that is already running and being used that does not need any (unknown) repairs. good luck.