The white stuff on the engine and the carbs is oxidation. Simple Green is pretty acidic stuff to aluminum carb bodies if left too long. Some carb cleaner, and some rags might help return them to a clean, dull grey.
First order of business is to pre-soak the spark plug pockets with WD-40. While that's happening, check the oil for contamination of moisture or water. If none, you're lucky. Whether there's a cover on it or not, moisture builds under the covers and will infiltrate your engine. Once you get the plugs out, and while the carbs are off, spray some WD-40 or pour some 50/50 mix of Automatic Transmission Fluid (ATF) and Acetone into the cylinders. This will spoil your oil, but it is very protective to the internals should any rust have accrued. It will dissolve the rust so you can turn the crank over to insure the pistons rotate freely within the cylinders. This you will do by:
Remove the left side front cover, the round one concealing the stator. Support the stator with a bungee or something, and under that cover is a 14MM bolt. Use some leverage and rotate the crank, make sure the bike is in neutral. If it's stuck, do your best to move the gear up to 4th or 5th (may have to raise rear wheel and spin chain while shifting up) and then put the bike on the ground and push it. This "bumping" can free the engine, especially with the aide of the penetrating oil and removed spark plugs.
Be warned: while doing this, the oil may comm me squirting out of the plug holes and cover anything in front of them, so like a BB-gun, only point it at something you intend to shoot
Of course, all this is based under the premise that the engine may have become stuck. If it rotates, then drain the ATF mix, change the oil, filter, and install new properly gapped plugs. Those rubber boots for your carbs may have become quite brittle and hard while sitting, and there are numerous tricks to refreshing them. Be certain the carbs are fully seated and properly snugged to avoid air leaks.
From the looks of it, I'd also start soaking the bee-jeezus out of the exhaust flanges with WD-40 to aide in removing the pipes. Same for the forks at the triple tree.
There's huge amount of threads with carb rebuild pics here too. And here's a parts fiche for your bike. Bookmark it so you can tell how and what pieces you need, or the listing for them. Not necessarily the place to buy them, but it's a great resource.
http://www.cmsnl.com/honda-cb500-500-four-k1-1972-usa_model460/partslist/Lots of really great members your way too. Welcome to the madness!