Wouldn't be too worried about it.
Most of the time I left mine off so I could oil the rockers and make sure there was a pool of oil still in there.
No oil runs down tot he engine so dirt is not a worry. Way the covers fit they wouldn't keep anything much out anyway.
Here we get mud wasps which want to block up every hole and cavity they find and get under things. They would love to get under that cover and make their dirt nests but they wouldn't go near anything open and exposed. I put a plastic bag over the top and tape it best I can to keep everything out when the engine isn't being used..... as in the last 4 years. :0)
Don`t start me on mud wasps, too late, you have.
Little bastards get in everywhere. If the mower or chainsaw won`t start it`s probably because the exhaust is filled with mud.
Last time I fired up my bench grinder nearly killed me, 2 pounds of mud let go at 2400 RPM! Apparently we are living through
one of the worst mass extinctions in global history, these little buggers need to be next on that list.