The subframe is aluminum. Depending on how much you will spend on welding, you can find a good used OEM subframe for around$100 bare OR upgrade your bike to the single bolt seat and new SX subframe and plastic. Looks much nicer and allows you to run a EE soft seat. The old style two bolt seats were pretty cruddy on long rides and the SDG built EE seats weren't much better.
let me know, I can put you in touch w/ a buddy of mine out of Michigan that aprts newer generation bike sout all the time and can set you up w/ a whole new rear section for pretty decent price