In mugen docs directory there are a lot of help, tutorials, faqs, etc.
I think you should make the sonic sprites from sonic battle sprites. Scaling 'em up (resizing, if you dunno what is scaling), fix the errors what resizing caused, change a bit on shading, to fit for jazz sprites, and it's done. I think you should post your plans to MGBR, maybe somebody will help. Give it a try.
Personally I can't say, I can help, I have already a lot of WIPs, and it takes a lot of time to work on them...