Ford D Series Help

I have slipped up badly here and I think I am going to need a bit of help from one or more of you experts out there. I bought a 1980 D Series horse box on Ebay. It looked good, was
driveable, and in the photo, it appeared to be just a little bit larger than a transit. When I arrived to collect, I found it was a monster (by my standards). I haven't driven anything that size since I was in the Army over 30 years ago. It scared the life out of me, especially on some of our narrow country lanes (with no power steering and air brakes that would stop on a tanner). All I want to do is move a horse and a pony, not half the Household Cavalry, so, my first priority is to move it on and find something a little smaller - and here is the technical problem. It drove well for the first 80 miles. Then the engine started hunting a little under light acceleration. Then it seemed to lose power particularly going up hill. Gradually it became worse. My first guess was fuel starvation, since it would tick-over fine and accelerate normally up to a limit - it was as if there were a rev limiter in operation. I dipped the tank and there was plenty of fuel. I spent the next 20 miles at 20mph in 3rd gear and every time I tried to accelerate beyond this 'limit', the engine would die. Is it a blocked fuel line/filter, or is there something else that might cause these symptoms?Where can I buy a new fuel filter and other parts? Thanks.
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Start with the ignition coil. Here's a hint... since that is carbed, fuel starving would be slightly delayed reaction on acceleration, given it runs okay at less power; coil secondary breakdown is immediate.
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