Did it engage into F while running? You may have to slide a small screwdriver into there so you can actually activate the linkage. Does it grind or just not budge at all when trying reverse? Reverse is normally harder to engage than forward is by hand. It sounds like you may have the shift linkage cable going bad. The way you can tell is that if you adjust it really far one way it will shift fine into one gear and not the other. It loses that"sweet spot" to shift into both gears as it stretches out over the years and has too much slack to perform both ways.
How long have you had this motor? Worst case scenario is that your clutch dog in the L/U may be worn out of service. I have seen in the past, where people will wear out the forward side of their clutch dog and then flip it over and keep the "bad" side facing reverse. Normally forward will wear out first. So if you know that this has not been performed we can more than likely rule this scenario out. (I think "flippin the dog" is a ridiculous practice, because the cost of a new clutch dog is minimal considering the labor required to remove it.)
