If you want us to determine whether it's truly a Rossin, then we'd need close-up photos of the seat cluster, the bottom bracket area, the drops and the headtube... basically all end-points of the tubes on the frame.
Most crucially, we'd need a photograph of the underside of the bottom bracket. The serial number should be clear and readable.
With that info, it should be possible to work out what it is.
As for what to change... assuming you like the frame and want to just do little things to make it work as a singlespeed, then I'd swap the bar tape to black first thing... I'm getting conniptions just looking at the photo ;) I'd also swap the stem to one with a steeper angle, the current one looks quite out of place. Those little things would bring the front-end together more, which would help with the looks immediately.
If you want us to determine whether it's truly a Rossin, then we'd need close-up photos of the seat cluster, the bottom bracket area, the drops and the headtube... basically all end-points of the tubes on the frame.
Most crucially, we'd need a photograph of the underside of the bottom bracket. The serial number should be clear and readable.
With that info, it should be possible to work out what it is.
As for what to change... assuming you like the frame and want to just do little things to make it work as a singlespeed, then I'd swap the bar tape to black first thing... I'm getting conniptions just looking at the photo ;) I'd also swap the stem to one with a steeper angle, the current one looks quite out of place. Those little things would bring the front-end together more, which would help with the looks immediately.