I am looking into better ratio rockers, but as far as I can tell these 4.0s ohv have a few things working against them valve sizes, rocker ratios, and compression ratio. Have the machine shop check your clearances(as I have yet to have mine that far apart yet) to see how of the head could be milled to get it around 9.5/10 to 1 compression. Increasing flow and increasing compression means power. The sohc 4.0 is running 9.7 comp and 1.8/1.5 valves which is pobably the largest factors for its 207 hp rating.
I did some checking for you, and the oldest I could find was k&n, they have an intake for a 96 4.0, they had a piture, and it looks pretty basic, may have to do a little fabbing, that or get one of those generic kits from ebay. It's hard to find performance stuff for the older rangers. The good news is, Morana v6 racing has developed a 4.5 stroker kit for the ohv 4.0, that should be an awesome product.
I just put together a "cold air" intake for my 92 4.0 using 3" intercooler pipe from ebay I used one 90 degree steel elbow and a 45 degree elbow. I just had to trim the pipes down to fit, no clearance problems at all. the fit on the stock throttle body was a little sloppy fixed by a tight clamp, and I am using a v8 maf body with the v6 sensor on it, with the t/b bored out to 70mm using a v8 butterfly. The aircleaner was just a cone type that clamped to the maf.
I have compiled a list of modifications for the 4.0 and am soon to finish installing them and have it dyno'ed I will be posting the cost and descriptions of them soon, should equal at least 225hp n/a for a reasonable price.
you should be allright then, the law states here, that the swapped in motor must be of the same year as the vehicle, or newer, they were using the 5.0 up to 01 in the explorer. To be legal, I'd have to swap in the 4.6, thats a big motor as it's a cammer, to much hassle, I'm just gonna build my 4.0 cammer, it has as much hp stock, as the old stock 5.0 mustang, "well", allmost.