Remove the rear wheel, then measure the distance between the inside faces of the dropouts, where the axle locknuts were just touching. Probably 120mm or 126mm. If 120, you can spread the frame a bit and put a more common 126mm clincher wheel in there.

There's a 6 speed in there now, so it may already be 126, or the previous owner put a 126 wheel in 120 spacing (common "upgrade").

I race on tubulars, but use clinchers the rest of the time. A few ride tubulars all the time, but commit to gluing or taping and learning about them.