Doing the white spacer mod from the top, definitely doable.

Spraying carb cleaner all in there, I would tend to say no to that, but maybe OK to WD40 or something not as caustic as carb cleaner. You don't want carb cleaner sitting in there against anything non-metal as it can eat at it, such as the tip of the float needle, plastic float parts, etc, AND most importantly the slide diaphragm.

be VERY careful not to mar the side of the slide, and when you put the slide back in make sure to clean the carb as well as the slide VERY well with a clean cloth so there is no oily residue or fingerprints that will attract oil and dirt resulting in a sticky slide.

When you are removing the screws for the white spacer mod, make sure you are careful not to strip them.  They are JIS screws, not American phillips.  What that means is a standard phillips driver will not seat all the way down exactly flush in the screw head and this gives the phillips a tendency to ride a bit high in the screw head and jump out and strip the screw head on super-tight screws. You should be ok, just be aware of this and give adequate down pressure onto the screw head when breaking them loose.

This goes for the external carb screws as well as those int he slide holding the spacer in. Some of us have replaced all of the screws with allen head screws for this reason.

JIS tips
Not #2 Phillips

But be sure to support the lip of the carb before you start banging away.
You're gonna need a long, Skinny one to get the tiny screw down in the hole.
Pay attention to which side is up.

I ordered one of those similar JIS sets. Hated to spend the money but I have used them.

Try ACE Hardware (if you have one) for the washers. They usually have more of the oddities and special things that the other stored don't.