I think a rewrite from scratch is called for.
If you make it BSD or MIT, then you can incorporate it into GPL and there are no issues, and you can incorporate it into GPL V2 and there are no issues, hence I would sort of hope you'd make it bsd.
Or look at the virtio include files in linux, which have a license that similarly lets you incorporate it into any license. I even pulled them into Plan 9 at one point, and that license used to be a mess
ron