the svn2git ruby gem is the only good way.
I did it here:
https://github.com/Foe-Services-Mirrors/ISPConfig3
but I did not remap the users like described here:
https://github.com/nirvdrum/svn2git#authors
If you like I can do the proper export. You need to send me a ssh cert you set as deploy key on github for the organization.