We have this problem with every sport, it's difficult to compare across eras.
If the two of these men fought in their primes Ngannou would murder him. That's not to take anything away from Tank or what he accomplished in the sport, but it's a different ballgame now with an entirely different level of training and athleticism.
Could Tank take his natural punching power and train like a freak and become a legit threat in modern MMA (if he were born at the appropriate time)? Sure. But as it stands, his prime from the 90s versus Ngannou's current prime? No contest, bet the bank that Ngannou will lay him out.