I backed up my ipad and then did a restore but now some of my apps are coming up incompitable?
The best way to do this is to connect the iPad to iTunes and do a 'sync' of everything, including its applications. This saves the iPad's applications to your local computer.

After doing the sync, you should then do a backup. This will save its settings.

The reason why it is important to do the sync is because there is the possibility that some applications that you had on the iPad are not available on the App Store any more. Imagine, for example, that you had Flappy Bird installed on there :)

After doing the restore, you should then sync the iPad with iTunes again - this will put all the content that was on the iPad before you restored it, back on the iPad.

If you do not do this, the iPad will only be able to get any applications that are still available on the App Store now, and any apps that are no longer available, will not be on the iPad any more.

EDIT: Apps coming back as being "not compatible", may be apps that used to be compatible when they were created ages ago, but have not been compatible with the original iPad for a few years now, and there is no longer a compatible version on the Store.