Re:
That's nice and very similar to mine. The fact it has original paint is rare. Most old Hetchins that have seen some use look like that and end up being professionally restored. Not many get left with the well used look. Once that original finish is gone, you can't get it back. Cheapest parts to use are those which you already have that are still serviceable.
Found myself in a similar quandary. Bike totally original but everything totally worn out. First idea - 1950s Simplex dérailleurs. The result was a bike fitted with parts that looked new, but didn't actually function that well. It didn't want to change gear and made a right racket. I spent quite a lot on these parts, and I refused to stump up for a Juy 543 which are quite frankly silly money, to get 5 functioning gears. Yes you can ride 3 speed but I like my knees just as they are.
Plan B. Sacrilege - Bin the cottered cranks etc and update the whole thing with 50s / 60s / 70s Campag. It works in a modern fashion which I can understand and set up and I can take it out for an 80k or so ride and hopefully it won't self destruct. Some of the cheaper old Campag stuff doesn't look a whole lot different to the posh 50s or rare parts. It will get the bike back on the road. This still costs a few quid.
Key thing is preservation of the frame. A gentle clean, avoid the decals if possible - they disentigrate. Cotton buds and something like car polish works well. My decals came off on your hands if you touched them, so I sealed them along with any lifting edges of paint with One Shot sign writers lacquer. Only do this of you absolutely have to. It is a one way ticket. If they will stay there with a bit of wax polish, do that instead. Steel wool or polish on what's left of the chrome, followed again by wax polish. It will seal the surface but can always be removed. Internally, frame saver or other corrosion inhibitor. Work slowly and on small areas. One tube or lug at a time.
Don't throw any of the original parts. Pop them in a box and tuck them away. You may have a change of heart and funds in the future and want to restore to original spec. Ripley cycle jumble is worth a visit for period bits.
Sorry for the ramble. Good luck and enjoy your bike.
