Hi,
Just replaced mine with Kenwood 6" splits.
The six inch speakers (round) fit in just fine with a little trim "trimming" and give much better bass response. Did not have to drill new mounting screw holes, screwed in just fine to the old holes with only a very slight angle to them. When selecting speakers try to get ones that do not have covers/grilles that protrude too much as they can get a kicking.
Fitted the crossover unit behind the kick-boards in the foot-wells and the tweeters were mounted at ear height on the "A" pillar using the top most trim screw so no cutting or drilling. Just took the little cap off removed the screw, fed the wire out and screwed them on.
So the advice is - try an upgrade to a slightly larger speaker even if you fit two way co-axial 6" speakers you will love the improvement.
Guzzi