Very different beast. The only thing that's the same is the clamp diameter, so they both clamp on the same intakes. But all the jets slides needles gaskets and seals are completely different.
The sachs "square" bings are prone to leaking and take more expensive harder to find jets and are more limited on tuning. The tough thing is the puch "round" bings are taller, or rather the bowl is deeper and so it wont fit on the sachs without hitting the engine, so to use a round bing you've got to spacer up the intake. If you've got a C ir D square jug on the sachs, it's easy- the intake is flat and you just need to cut a piece of smooth cutting board or particle board or whatever to get maybe 5mm extra lift. With the A and B theres an angle to the intake that makes a riser just trickier to make, to drill the bolt holes at the right angle or whatever.
Either way, what I would do is get it and try to repair the stock square. Maybe get the doscycles.com sachs rebuild kit, it includes all the carb gaskets ya might need. Most likely the square float bowl gasket winds up being the needed piece if its leaking. If the square carb's entirely missing or very broken, make a spacer and go with a puch carb on the stock intake. The stock intakes unless it's a B jug are 14.5mm so up to 15mm round bing is a good fit.
Much more info in the wiki if ya search 505/1a performance- most all the info there is equally applicable to the 504 or other C, D sachs motors