For example a modern EICAS system is required today, and all modern passenger aircraft have one. Except the 737 Max.
The 737 Max 7 and 10 had to get a waiver due to not being certified in time by the hard requirement to have one when updating old types. Let alone certifying new types.
Considering the low ground clearance is one of the major issues of the 737 today (which lead to the whole MAX disaster), you'd have to replace the landing gear, and with that you'd also need to make changes to the airframe itself.
What are you basing this on?