This is a very costly undertaking. The block typically does not go down to the ground and there is no footer to support the wall being lowered. You would have to support the house, excavate/dig for a new footer, build the wall and then excavate the center. I have heard of this costing more than rebuilding the house. I have the same issue and have thought about possibly excavating the center wide enough to walk from end to end. Building retaining walls to hold back the dirt then concreting the remaining sides. Essential a pathway with a concrete shelf on either side. I could excavate by hand although time consuming and I only need to go deepbenough to stand. Its the retaining wall that could be fun. I would remove one block from my basement walls and shoot the voncrete down.
You must also be getting christmas decorations out of the crawlspace at this time of year. I have cut hundreds of doors and openings in concrete/block walls, thats the easy part.