Bruiser,
According to the
RCCA rules, any plane legal for 2105 is also legal for 2610. It looks like the only difference is the size of the engine ... .21 for 2105, and .26 for 2610.
The RCCA racing class rules specify that 2610 legal planes are to be used, therefore 2105's would qualify as well.
In terms of mounting bolt patterns, I'm not sure of any .21's that have the 1.5" x 0.59" pattern that many of the 25's use. John B can probably advise on this issue.