SteveT.
100cc
As per page 10 of the manual found on this page, you use wood screws. If there are already holes there (and I assume there are, as it is used), if they are not already stripped out, give them a drop or two of thin CA, wait for it to cure, then just screw the motor on. If they are stripped, perhaps you can rotate the "X" mount a few degrees and re-drill...
SteveT.
SteveT.