The county courthouse where the property is located will have the records, even if not yet digitized. You'll just have to actually do there most likely, to research and find it. I would fully expect the county to have at least the purchase price from back then. If they also have the property tax records that would help to. But in figuring allocation percentage you can use the current tax records for that, and I doubt the IRS would have any issue with it.
The important thing here, is to make the attempt and be able to prove you tried should you ever be questioned on it. When all else fails, a "guess" that can be supported with things such as the cost of compatible properties in the area can be utilized. But guesses are an absolute last resort.
