How to restore - my way.
#1 - get it running. https://mopedarmy.com/wiki/Fred%27s_Guide Fred's guide will walk you through about all you need to know. With it running, even slow, you'll be able to diagnose OTHER problems like brakes, ignition, lights, etc so you know what you need to fix. get the trans all filled with good fluid etc etc.
#2 - you can rebuild that one or adapt another engine, kit etc. all depending on what you want out of it and how you felt with it after step 1. At minimum I'd recommend swapping to an A cylinder, intake (should bolt on) and exhaust header. A dellorto SHA should fit allowing more tunability and reliability from the carb, and an A header with slip-on chamber gives a modest performance boost. Also plug the decomp. Performance will be up to you ultimately and everything you need to know including plugging decomp is here: https://mopedarmy.com/wiki/Sachs_505/1a_performance
#3 - general maintenance. whatever you decided to do with the engine, a few things that are almost always a good idea - new tires for sure, new cables usually good idea, and new brakes often needed too.
#4 - aesthetics last. I wouldn't worry about the side covers, I think it looks better without. The floorboards are hard to find but stull like that post a "WTB" post (want to buy) in the Buy/sell forum here, also try "lovers of sachs mopeds" in facebook groups.
#5 (and really should be 0.5 since this probably oughtta come before #1) - find some local folks to ride and fix shit with. it's the best part of mopeds hands down