I wonder

Well about that...

The Maus' frontal armor is 200mm sloped at 52°, giving it 360mm of thickness. The turret is less at 240mm.

Now, let's take a "low-end" modern tank, the T-62. And take a look at the penetration values of it's APFSDS for the 115mm 2A20 smoothbore gun.

The most capable ammunition for the T-62, the 3BM28, is capable of penetrating, from 500m and an impact angle of 60°, 377mm of rolled homogenous armor, so yeah...

There's still a chance though, the 12,8cm KwK 44 is capable of penetrating 260mm of RHA from 500m at 0°, the turret face of the T-62 is only 200-220mm.

but what if it was modernized with all the necessities for survivability that we have today including the armor as well

Well, the Maus is already heavy as it is, but there are some ways,

Without any major overhauls, like majorly restructuring the interior of the tank, replacing the turret, reshaping the hull, etc, the Maus can be fitted with some fire control upgrades, like a laser rangefinder, wind sensor, thermal and infrared sensors, new munitions for it's 128mm cannon, etc, this can improve the firepower and accuracy of the tank with some sacrifices to mobility, however that's pretty much the limit to modernizing a Maus, it would still be deathly slow, made even worse by the addition of all the sensors, the turret is also still dead slow, and the armor is still ineffective, even against the earliest variants of T-64s or Leopard 2s.

If the Maus were to be upgraded with a major overhaul however, well the result would look somewhat like this.

New modern 120mm L55, Commanders Independent Viewer (CIV) with it's own 25mm autocannon, ballistic computer, new power plant, armor replaced with composite armor making the Maus lighter + composite armor add-ons, Explosive Reactive Armor (ERA) hard-kill Active Protection System (APS), thermal imager, infrared sensors, and smoke dischargers, if this isn't a modern day Maus I don't know what is.

If the maus was the E-100, then that would be the E-110

maus added to future war plans