Well, I even assembled and sold some Raleigh 3 speeds back in the early 70's- there's certainly nothing that is sacred about STEEL 1 3/8 x 26 tires. They are harder to find, offer no choice of width and STEEL rims that are obsolete otherwise. The blessing here is that 700c wheels and tires WILL fit, and allow more modern and shorter lightweight alloy brake calipers. The spokes and nipples probably have their fair share of rust. Like I said- mounting a sprocket on a three speed hub is an extra affair, and if you succeed, you've spent time and effort on very old hub with complicated internal parts more prone to fail.
Fenders on motorbikes are nice only if you plan on riding in the rain- I avoid that myself, and mainly for the bikes sake. On a motor build they are usually in the way clearance wise and often have to be cut up for the chain to clear.
That only leaves the frame and bars and cranks- cotter pins were a hassle back in the 70's even when all three sizes were available, and the tend to go bad quickly again, leaving the VERY heavy steel 3 piece cranks to do that irritating slipping bit when the pins wear. A modern single sprocket crank is much much lighter and not that expensive, plus you won't have to adapt the bottom bracket otherwise to replace. The old steel chain wheels were oversized for the 3 speed hub- 48 teeth or more, so without the hub it's going to be heavy hard peddling. The rubber pedals are HEAVY and wear out quickly- there's a world of alloy pedal choice that's come along since
These are the changes that people seriously still riding internal hub bikes usually opt for these days. The hubs can have 8 internal gears- but I've never ridden one.
With a small 50 or electric motor, you could set up a bike that appears similar to the original 3 speed, but is significantly lighter beyond the original, before motoring. Maybe 20 25 less weight.
alloy bars and stem would help too and be similar otherwise. Personally I've always gone with alloy BMX bars and stems for even more height, and still get great speed to have a lightweight and very peddable moped otherwise.
the rod brakes are quaint and cool for an antique, but better and lighter brakes and levers are available , and your life may be riding on them otherwise
It's a good frame otherwise and if you just want a restored 3 speed bike, then do that.
If I could do a dream machine- I'd put a 33 cc China girl on it - for even LESS weight and LESS noise- with an under bar PLASTIC TANK and cap- one liter MAX- for weight reduction., so the kit would add no more than 15 pounds and the bikes final weight would be about 40-45 pounds Total. Not an off road thing by any means, just an efficient communter as this was designed to be originally with the best technology they could muster back in the 1930's or there about when the 3 speeds were designed.
The basic idea is still the same- go as far as you can with the least effort.