I should think it will be fine. If it's been carrying that bitmac drive for 40-odd years, then there's absolutely no point in ripping it out and replacing it with something new. You know it's not going to settle or move - not after all this time!
You may have to re-grade it to accomodate the different levels of the block paving. Most old bitmac drives are 75mm thick, whereas you need around 100mm to accomodate the bedding and the blocks, so scraping off the top 25mm or so and re-compacting is a likely task.
Also, if there are any serious voids in the old sub-base, it's well worth getting in a couple of tonnes or so of extra bedding sand (coarse grit sand), scattering that over the surface and wackering it in to fill any holes before placing the bedding layer.
Hope it goes well for you - keep us informed of how you're doing.