Well, to balance out the "keep it original" here, I do not agree that changing a 425/435 to an 602 is the same as "wrecking" the car. C'om on folks...

The 602 doesn't share parts with the smaller engines, ok. Obviously not the same, but in all fairness, they are similar. It's a matter of definition, and not so important actually.

Of the tree alternatives, the BMW, the 602 and building a 425/435 into a "602" or what ever..... Just using a 602 is CLEARLY the cheapest and easiest solution for more power. And regarding the question in the first post, is it hard to do, that's impossible to answer because if you're used to 2CV mechanics and have mechanical skills, it's easy.
If you're not mechanically skilled, and think changing a clutch is difficult, then it's difficult. So it depends!
And btw the car isn't more "wrecked" than it can easily be built back to original specs later for some reason.
Do one need more power? It's up to the user to decide.
To each his own, I guess. But one thing is certain, I enjoy looking at un-original cars, being just small details or totally mad max insanity
The "make it you're own car" thing in the 2CV community is very cool, even though not all custom cars are cool..

+ 1 to that.

Its your car, do your thing.

I think you need other springs with the heavier 602 engine, for the rest not much problems.

Is it really any notable weight difference on the engines??
I would have thought not, maybe just some kilos.

The softer springs might be too soft if you use the loud pedal a lot on bad roads maybe.

Thanks folks for all of the input !

From my perspective.....I have zero issues with "upgrading". Something that looks old but with improved underpinnings is fine with me. If it can be returned to "stock" then all is good.

The Fiat 500 I did a nut and bolt restoration on, replacing virtually every body panel, and every nut and bolt with new had uprated engine and disk brakes, but externally was 100% original, I felt no guilt !

I've driven 602's for many, many years. No problem with performance. 425 is new to me, hence the original question....

I'm still considering my options so not yet about to "ruin" an original

You don't need to change anything on the body or chassis. You can use the early pedal arrangement with a later 602 engine and later drum-brake gearbox just by using the Oct67>May68 AK350 clutch cable and by adapting the front silencer so that it's left-hand inlet pipe runs a bit further from (and doesn't heat up) the brake reservoir and so that the exit pipe goes out the right-hand side, along with a modified (reversed) swan-neck pipe to the normal side silencer mounted on the right-hand side of the car instead of the left. The only other thing you might need to do, (depending on which carb you want), is rig up a pivot to convert the 'push' operation of the accelerator pedal to a 'pull' on the carburettor.

I have a 3 page document of how I did the 602 upgrade on an AZU Fourgonette van1962, I can e-mail a copy if you message me, because I don't think this site will accept a PDF file.