Adding a larger chain-set to a mountain bike

There are a few issues that you will run into.

The larger difference between the smallest and largest chainrings means the derailleur has to be able to take up more slack in the chain when shifting from largest to smallest rings. This may exceed the Total Capacity specification of the derailleur. The capacity needed is (difference between smallest/largest sprockets) + (difference between smallest/largest chainrings). If you increase the size of small and middle rings by the same number of teeth, the capacity in not affected (but a longer chain is of course required). You also need to check the max chainring size difference that your front derailleur will accommodate.

You may also find you have insufficient clearance between the chainstay and a larger chainring.

Presumably a 7 speed drivetrain has a cartridge bottom bracket. If you need to replace the while crank you may find you need to replace the bottom bracket as well as different crank models require different axle lengths.

Did you 90s bike have smaller wheels than you current one? Remember than if the Oder bike was on 26” wheels and you newer one has 27.5” or 29”, you effectively have a higher gear ratio anyway, so you may not need a 48 tooth chainring. In fact 662/559 = 1.1, 44x1.1=48.4.

Your front derailleur is spaced for +10 and +8 gaps between small and middle and then middle and big ring. If you examine your front derailleur you will see that it has a groove spaced to shift for those gap sizes. Going to +14 then +10 will give poor shifting performance, so it would be best to find a matching front derailleur, or to maintain those gap sizes, e.g 48/40/30

A 40mph speed on a flat road is not particularly plausible on a mountain bike. On a time trial bike in an aero position and tight clothing it is possible. The world record time trial time for 10 miles is around 36 mph @ 450W. https://cyclinguphill.com/time-trial-records/ Since power required to defeat wind resistance is proportional to the cube of speed, and proportional to the frontal area of the bike (CdA), it follows that even on a time trial bike you'd need to be putting out around 555W to do 40mph on a flat road. A MTB I would expect to have almost double the CdA of a time trial bike, so you'd need to be putting out 1000W minimum, which is unlikely.....

If you want to go faster you would be best looking at a bicycle with drop bars as a starting point, because fitting road-like rings to an upright bicycle is essentially a masochistic exercise, because most of that work you will need to do to push those rings will be lost to air resistance, and you will end up in top gear at a very low cadence, which won't be good for your knees. It will be many times easier to pedal quickly if you find a bicycle that is comfortable to ride but minimises your body area exposes to the wind, and if you ride in tight clothes (i.e. lycra).

Another option - change out your cassette to one with an 11-tooth smallest cog.

Per Sheldon Brown's gear calculator, pedaling at 100 rpm on a 42-tooth chainring and a 12-tooth cassette cog results in a speed of 28 mph. Going to a cassette with an 11-tooth small cog, at 100 rpm that's 31 mph. Getting up to 120 rpm results in a 37 mph speed.

And 37 mph on level ground is really fast on a bicycle - as in "speed reached in the final sprints in Tour de France stages" fast. 30 mph on a bicycle is fast, too. If you're not struggling to reach 30 mph on a bicycle on flat ground, you should be riding in the Tour de France. An average-size rider has to sustain a power output of 400 watts or so to go 30 mph - while riding an aerodynamic time-trial bike. Sustained power output from cyclists who have never raced rarely exceeds 150 watts.

40 mph on level ground while riding a mountain bike with 26" wheels? You would have had to have a 48-tooth chainring and and 11-tooth cog and be riding at 120 rpm to do that.

So if you hit 40 mph on that old 26" MTB, you should be able to hit almost that fast with your 42-tooth chainring on your 29er just because of the larger wheels - as long as you have a cassette with an 11-tooth small cog.

But at the end of the day, bigger gears do not make you faster. The speed you can sustain depends mostly on how much power your body can generate. The biggest limiter after that is how aerodynamic you and your bike are.

Truth be told, the derailleur can handle the larger chainrings up front. However, you would have to bear in mind that each front chainring would be limited to only a few gears at the rear to prevent the RD from over-extending its pulley arm.

Naturally, I understand that answers here should err on the side of safety and practicality, so it would be much more cheaper to replace your cassette with one that has an 11T cog which would allow you to realize the same/faster speeds without changing your front chainrings.