Hey OP,
Unfortunately if you cannot find an exact replacement, there's no easy way to just eyeball a matching driver. Too many variables, which would require a new crossover. I believe you have 2 of these mid-woofers, right? You'd need to replace all 4, and get a crossover matched to the replacments, IF you found drivers that worked well in the cabinets.
You are well into hacking here and unless you are looking for a DIY learning experience with a steep learning curve, strongly suggest avoiding it. If you want to DIY, then DIY instead of trying to back into pre-existing tweeter and cabinetry.
Best,
E