In most places in the US and Canada, it is not legal to repair a compressor tank yourself. It must be done by an authorized shop (Natn'l board "R" stamp holder, or equivalent). Non-commercial users can get away with it, but if there is a failure, your insurance probably won't cover any damage or injury.
I still maintain that you should be able to put the compressor in this space without resorting to all of the suggestions made in this thread to modify the tank drain which are iffy at best. If you cannot put it on a shelf as I suggested, then build a weatherproof enclosure that has vents to allow air in and heat to dissipate while keeping the rain out as well. When in doubt, KISS!Because the lean to is concrete block on three sides. I guess a more accurate term would be a block closet. I'd be left hanging one end of the compressor out of the roof line if I left it laying long ways.
The tank drain could be an issue. IF you're good with copper fittings & ferrules you could insert a dip tube to the lowest point via the drain valve opening & then put a petcock or valve on the other end to get the water out of the tank. Otherwise, go for it. :thumbs_up