Can the left hand crank seal be changed from the outside, or do the casings need to be split?

It needs taken apart. However, if you are tight for money, check out the local parts store. They have products that may help slow or eliminate the oil leak. They make things that you just add to the oil and it causes seals to swell. This isn't the way I would recommend to fix it, but it may get you by until you have the money to fix it right. I know what it means to be tight on money. I hope it helps. Make sure they sell you something that is worth purchasing and not a cheap usless product. I'm not allowed to recommend brands.