That sounds not too good Weggy, if you can now turn the engine over with the kick start that's a plus I think.
I think your next step is to remove the end cover from the gearbox and have a look see if you can see anything a miss, if everything looks fine then remove the inner cover. take photos as you go and ask, ask ,ask whenever you come on anything your not sure about.
As the main shaft key sheared it kind of points to something locking up in the box, as you can now turn the box over with the kickstart perhaps wht caused the problem is lying in te bottom of the box so drain the oil out into a clean container and watch for anything lying about in the box.
The gearbox is fairly simple even though it doesn't look that way if you never been in one before so don't be put off, plenty friends here to help provided you have some pictures to help
Keep us informed of progress