I'm also running the SV2 on dirt and never had that problem.. Just make sure that there's a tight fit where the bodyshell meets the gear cover so no dirt "falls" into the gears as the gearcover doesn't cover the pinion.
A more simple solution than making a new motorplate is just to glue a bit of an old foam on the inside of the bodyshell so no dirt can fall between the shell and the cover..
//Kenneth
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