One option is to cut off the existing axle and use a bolt on axle such as http://www2.towerhobbies.com/cgi-bin...?&I=LXD828&P=0

If they have a separate axle that can be unbolted and replaced with a longer axle such as http://www2.towerhobbies.com/cgi-bin...?&I=LXD824&P=0 .

The only problem you may have with doing that is if the plane has retracts - the wheel wells will need to be modified (bottoms cut out ) to accept the greater depth of the wheel/axle.