That's the reason OHC's will never grow in popularity like OHV 2V's or X-flows are in the US. Parts supply.
With a 2V or X-flow, if you run a bearing or need a distrubtor cap, you can make a US bit fit. If you drop a rocker arm, or a valve guide, the US bits can be made to fit. If you drop a valve or pushrod, the US bits fit. Strip a head bolt, US bits fit. If you want to run a carb, a Holley or Holley- Weber or Autolite fits right up.
When you look at OHC's and DOHC's, you can't make a US bit fit. After 1986, the electronics became a kill joy gate keeper with even the unleaded 3.3' and 4.1 X-flows. With the OHC's, there isn't one bit that's interchangable except the conrods. The BTR gearboxes have the starter on the wrong side, and no-one is helping out with a good UltraFlow 4-bbl carb and distributor conversion because all the aftermarket wants to sell you is a race omputer system which requires things to go with things to make it work.
In addition, the ignition and injection stratergies of most aftermarket instillations are inferior to the stock Ford EEC set-up, and there are no options if the engine brakes down on the freeway. Crocadile Dundee's knife aint gonna help you.
The OHC has a valve train and instillation space issues. Cam, springs and profiles that work better than the stock XR6 exhast systems are not easy to get. The aftermarket is not sharing the important variances between EA, EB 1 and 2, ED, EF 1 and 2, and EL engines. The EL XR6 engine is so different to the EA, they are superficailly the same, but the breakout codes only tell you what part you have to order from Aussie. Ignition faults are very common, and there is no support from the factory past 10 years. EEC units prevent full cam upgrades on later OHC engines, and the good bits from XG XR6's which allow the Aussie OHC's to fit inside American Mustangs and Mavericks are thin on ground.
The Neanderthal X-flow has been getting better support from Aussies, but even with 600 000 engines out there, its still very much a risk for an American to build one up. The only saving grace is you can use other pieces without having to pay silly money. Transmission compatiabilty and great cams and induction set-ups are easy to get. It's only past 4500 rpm that an OHC really shows its worth, and the low end torque of an X-flow is at worst 20% less than the best XR6 engine. Specific fuel consumption on OHC engines isn't that great, and most come in tandem with the BTR automatic and transmission controller. Lots of little things to thwart progess.
