You can fix your skin wto work in DNN 9 normally, although the Menu could be an issue.
Solpart Menu has been removed, if that's in use, replace it in the skins ascx files as described here:
The other thing you need to do is fix your containers (the ascx files).
Any reference to solpart should be removed and they should continue to work.
Please make sure that if you change your ascx files, that (if they exisit) you remove the correcponding htm files.
(or you risk overwriting your changed ascx files when DNN tries to parse the htm files)