I installed the correct dust cover however it seems that because I also installed a mini rear reduction starter there is still a small opening not covered by the dust cover through which I can see the flywheel teeth. Will this be problem in terms of water and road dirt getting in?
Well, I wouldn't worry about it. I whole bunch of my flywheel's teeth are exposed.
I would probably make up a tin cover to seal up the hole, or fashion a screen for it to allow some airflow around the clutch.
If not you could remove the dust cover completely.
The reason for that, is if any debris should enter through the hole. It has nowhere to exit. And it will circulate around inside the cover. Possibly causing damage.
Other than being unsightly, and a bit of a hazard. Not having a cover won't be detrimental. And it does help cool the clutch for some of our more "aggressive" operators ;-).
Live like you mean it.
Fred