Replace starter without pulling left cover?

I pulled the starter off of my 'barn find' KZ250 and have a new working one (hoping this was the reason it was parked and nothing else). I did so without pulling the left cover. The starter sprocket is still visible via the opening for the starter at the front of the gearbox. I would like to not pull the left cover if possible (worried about the gasket if there, creating a new oil leak, and some of the screws are pretty frozen and I don't have an impact screwdriver (yet)). I have the oil drained right now though.

Does anyone have any suggestions for being able to install a new starter/getting the sprocket on the end of the starter? Short of deliberately laying the bike on the left side to negate gravity...

I think I can run up to northern tool and get an impact driver in the morning - is that suitable for loosening the left side engine cover screws if they're particularly stubborn?

As always, thanks for your wisdom as I work through the challenge I've signed up for!

FYI, I went back out to the garage after dinner and gave the screws around the left engine cover a go… got ‘em all out with just a screwdriver. Toasted the gasket when I pulled it off, but I’ll order a new one. 

This was going to be necessary the whole time - the teeth of the new starter motor drive shaft didn’t fit the old sprocket, so I had to replace the sprocket altogether.. hard to imagine doing that without that left cover off.

Welcome to our brand of madness!  In addition to an impact driver, most of us recommend getting JIS (Japanese Industrial Standard) screw drivers to work on your bike to avoid ruining the screw heads and causing new problems.  Vessel makes good ones and they are reasonably priced.  Get a #2 and a #3 at the least.  You can find them online at amazon and others.  

Scotty

Z1Enterprises, a sponsor of this forum, sells a really nice set.

www.z1enterprises.com/jis-screwdrivers-s...three-144-244-4.html