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I have an old 1950's English bicycle with cotter-pin style bottom bracket like this:
I find the cotter pins fail fairly regularly and are hard to replace correctly. I was wondering if it's possible to replace them with a modern bottom bracket without having to do welding/major work?
I did this just last year. It's not a hard upgrade. You'll need to replace the bottom bracket and crankset. With a bike of that age, depending on the make, it might be difficult to find a modern bottom bracket that can fit your bike. Take it to the local bike store to determine the shell width, or measure with your current one. This requires being pretty precise, as there are various standards and they all vary by only a couple of millimeters.
Get a crank set that matches the number chainrings you want (looks like only 1 from the pictures), and is compatible with the new bottom bracket and you are set. Square taper is a good choice for upgrading old bikes as it's cheap and ubiquitous, and works great. A bottom bracket tool for putting the new cups in is quite inexpensive, as is tool for tightening the crank arms. You might already have something sufficient for this as it isn't a special tool, usually just a larger allen key, although you'll need a lot of torque, so one that fits on a socket wrench is advised.
If you are unsure about doing it yourself, your local bike shop can do it in 15-30 minutes, and shouldn't charge too much.
My bike wasn't as old as yours, but I'm pretty sure English bikes from the 1950s still used either standard 68mm British bottom brackets, or in the case of a Raleigh, a slightly different proprietary size. 68 mm is still the standard so you're in luck if you have that size. I'm pretty sure the Raleigh size is still available as they were quite a popular make of bike.
Assuming you can get the bottom bracket out (this may be non-trivial), Velo Orange among other companies make "universal bottom brackets".
To quote Velo Orange: "Grand Cru threadless bottom brackets fit most frames, even those with Swiss threading. And they work on frames with damaged BB shells, even if the existing threading is totally stripped.
Unlike previous attempts at threadless BBs, this new design is "internally expanding". As the adjustable cup is tightened, the silver sleeve (see photo) is pushed up the tapered alloy cups, expanding and locking in the BB. This differs from older designs, like the Mavic, that pressed in from the outside and would sometimes slip. Installation requires only a traditional-style BB spanner or a pin wrench. The BB shell does not require chamfering or facing; simply slip the BB into the shell and tighten. It is easily removed and existing threads may be reused.
Like the other Grand Cru BBs, these have sealed cartridge bearings, so they require no servicing and should have a very long life. Taper is JIS. So far, we have tried these on British, French, and Swiss BB shells and they work perfectly. Customers report that they also work in Raleigh frames with a 71mm wide BB shell. These BB's won't work on mountain bikes, as their standard width (73mm) is too wide for the shell to expand. They do not work on Italian threaded BB shells because those have a wider ID."
Another thing to note is, if you can find the threading type, Phil Wood probably makes an appropriate bottom bracket for you. You'll likely be stuck with JIS for the crank, but theres plenty of new lower end cranksets or older higher end cranksets which use it. Note that Phil Wood is pricey, but if anyone makes the right BB cups and BB, it is them.
Here is some information from Sheldon Brown on the case of vintage Raleighs - he notes that you can combine Phil Wood cups with a Shimano BB.
A last dig option is to re-tap the threads for the bb to something modern, but this may be relatively difficult (and wont be super strong).
