It is a bit tricky, but it can be done. Make sure the fork pinch bolt on the RH fork is loose enough. Before putting the wheel between the forks, put the spindle in a bit on the RH side so that it stays inside the fork a little. The speedo drive should be greased, so should be the spacer. For me, I used to temporarily fit the speedo drive on the wheel hub, same with the spacer on the speedo drive. This was only possible with the grease. It kind of held together as a result of the factory grease.
Then using one hand, I used to lift the wheel, align everything with the hole next to the spindle and using the other hand I would force the spindle through. When it appeared that it had gone through almost all the way, I just made use of a mallet to get the spindle across fully. Then all you need to do is adjust the speedo drive so that the speedo cable isn't stretched when you turn the handlebar and doesn't scuff the paint on the fender. Followed by hand-tightening the big M16 hex nut on the other side, so that you can ensure the speedo drive is not being hampered. After this, you just need to tighten the hex nut properly followed by the pinch bolt.
Yes, you have to keep the wheel lifted with either your shear strength, or use a wooden plank of some sort to raise the wheel in order to give yourself leverage to align the spindle/speedo drive/spacer with the hole. If you have the front end supported by a house jack or some similar equipment, just lower the front end a bit after you get the wheel between the forks. Then hopefully it'll be easier to align everything.
I am sorry if I haven't been clear enough with my explanation. As wildbill mentioned, more experienced members may have neat little tricks to get the front wheel back on easily. Not sure my method will work out for you, but I've done it two or three times already without too much trouble. It surely is a bit of a struggle for one person to put the wheel back on. But doing this a few times will help you out, I suppose. And you will need to buy a new speedo drive, if it's cracked etc. Riding without the speedo working is quite scary for obvious reasons haha (has happened to me before)