The 750 bottom case can be removed with the whole engine upside down, no troubles with that approach. You will find that the crank will be a little tough to lift far enough to R&R that seal: the best thing to do is to remove the cam chain's tensioner from the back of the cylinders to give it the max slack the cam chain has at this point.
I've done it before, it goes about like this (and helps if you have another hand to help at that moment): pull off the side covers (tranny, clutch, sprocket, alternator) after draining the oil, and remove the 7 (some have
6mm bolts from the top side of the case, and the one nearest the countershaft (above it), this will be either 6mm or 8mm, depending on engine vintage). and the 3 8mm bolts across the back. Don't forget the 6mm bolt back by the kickshaft (like I often do). Then turn the engine on its head. Add 2 or even 3 pieces of 2x4 under the rear of the cases to make it a little more level to work with, then pull the 8 short 6mm bolts off across the front and the 4 other ones holding the cases snug across the middle. Last, remove the 10 long 8mm bolts along the crank's bearings (the 2 in the middle are longest) and plan on shining them up on a wire wheel, as they will likely be crusty.
With a long (long) screwdriver or short prybar, gently pop the cases by levering back by the countersprocket, and the boss on the cases back there. It will either gently pop (if still virgin sealant from Honda) or will just separate. If there is even one little bolt left, it will not separate and can break the cases, so be sure you have all those little 6mm bolts out.
Then the tricky part that needs a friend to help (or swear words, ether one works...): you will have to lift the crank straight up from its bearing seats in the upper case. It WILL NOT come out at any angle, it is well-machined into its home slots. Since your cam chain is still connected, you will only get it up loose about 1/4" or so at the most, which will just barely be enough to pry out the points seal. While still holding the crank up (I have used small pieces of 1/4" wood dowels for this part when working alone and using swear words to assist) you then slide the new seal into place and press the flanged back portion into place, then let the crank drop back in the bearings. The hard part is done.
Now is the messy part: you need lacquer thinner or acetone (or the like) to remove the old case sealant on all the mating surfaces. Then you put Hondabond 4 on all those same sites: be careful near the crank's outer 2 bearings so you don't end up filling them or their grooves with the Hondabond. I apply there with a cotton swab for better accuracy...then you have less than 10 minutes to set the cases back together (maybe more if you live in a humid place, here it is a desert) and make sure they seat. DON'T count on using bolts to "pull it closed", it will (and MUST) seat together without bolts. Then start the closing with the 10 crank bolts, fully torqued, and work from the inner 6mm bolts outward. I do the ones across the front last, then turn it over. Do the long 6mm top bolts from the center outward, too, and the last ones are across the back of the engine.
Then all you have to do is reinstall the engine!
