Replacing crank and bearing on cheap Murray

You have a frame designed for what's called an 'Ashtabula' crank. Details can be found here https://www.sheldonbrown.com/opc.html.

Apparently Ashtabula frame to English/ISO threaded bottom bracket cups are available.

RJ the Bike Guy has a video on how to do the conversion:

There are many levels of "upgrade". To be "better than the junk it came with" there are at least two options with many shades of quality level.

1. One piece crank upgrade
There are different qualities of one piece cranks / bearings / cones / races. Murry, Huffy etc. come with the lowest quality.
Back when Schwinn bicycles were made in the U.S. Schwinn bicycles with one piece cranks came with good quality crank parts. "New Old Stock" (NOS) Schwinn bearings, races, cones, and crank would be an upgrade to the parts you removed and they will fit your bike. Other sources for quality one piece crank parts would be made by Tioga, Tange or some other name brand maker.
As with anything there are advantages and disadvantages to a one piece crank. They are easy to maintain, they are very strong - they are also very heavy. This would be the least expensive upgrade.

Sheldon Brown has a nice over view of one piece crank types and interchangeability.

2. Three piece crank
The video Argenti has posted demonstrates how to replace a one piece crank with a very nice three piece crank and sealed bearing setup. There are a variety of solutions offered by different companies to accomplish this with a wide range of quality and prices. They will reduce weight but I would argue that a good three piece crank is no stronger than a good forged chromoly one piece crank.

As you convert your Murry into an e-bike remember that the quality of crank parts you removed are relatively equal to the quality of parts on the rest of the bike - including the frame.
Murry designed the bike to be affordable and to do that they compromised on the durability of the entire bike, not just the crank parts.
Adding an electric motor to a bicycle introduces forces the bicycle was not designed for. Using a low quality bicycle as the basis for an e-bike is especially problematic and could be dangerous.

As others have pointed out, you can get an adapter bottom bracket and then use whatever crank you want. There are some bikes in the world this can make sense for, namely those with good production values that happen to use one-piece cranks, which are uncommon by numbers but do exist.

In the more common situation of a department store bike that someone is riding, for better or worse, and something functional is needed for minimum price, the good solution is to get any chromoly 1-piece crank (approximately $25US today) and one of the fancier one-piece-crank bottom brackets, such as the Odyssey Dynatron at $15US, which are essentially designed for situations like this, i.e. good quality and actual sealing but still a drop-in dimensionally compatible replacement for any one piece crank bike. The package is still cheap but will be able to stand up to hard use and heavy mileage.

Well, it has been expensive, but I did it to learn to work on bikes. I could have bought a very nice bike to use for what I've spent to convert it. I took the old frame and had it powder coated. I used a bottom bracket converter with a Shimano bottom bracket. I have literally replaced every single thing on the bike. I haven't done the brakes yet(or shifters), but they will be mechanical discs shortly. One interesting thing about this old frame. The fork was a 1" threaded fork which you supposedly can't convert. The 1 1/8th headset and fork fit perfectly in the frame.