Fixing attic conversion, timber or rsj?

I'm looking at trying to fix up my attic conversion. Current beams are timber and only 5 x 2 I think. My da reckons this is causing the roof to sag and to replace with an rsj. I was quoted 800 wuro for the steel which is a fair bit.

Could I just use 9 x 3 timber instead? Would be difficult to get in in one piece so would splitting it defeat the whole exercise?

Any help appreciated!