I was looking in to a spark arrestor also, found out the Kubota
one is like a muffler and was told it adds exhaust restriction
found out local dealer was using a john deer screen type
less restriction and cheep fits on end of existing pipe. In northern calif where I live if you start a fire you can be charged to put it out. The local dealer said it was required
by the calif division of forestery and probably is by USFS
Either way if you're going to mow in a dry field it is probably
a good idea to use a spark arrestor at least while you mow.
Also I have seen fires started when people mow a field on a
hot dry windy day and hit a rock they don't see down in the
tall grass. So be very CAREFULL mowing or it could be a disaster and VERY expensive