I would inspect them and see if they still are "fresh" enough i.e. not hardened and smashed down. It is not real complicated to measure the radial gap of the two parts being sealed. Measure larger diameter and smaller diameter being sealed at the o ring grove (if there is one). (OD-ID)/2 =radial gap to be sealed. Next measure with a caliper the o-ring thickness. Now is the complicated part. If this is a forced pressure line, the pressure on the o ring has to be higher than the pressur eit is sealing. The pressure on the oring is computed from how much deflection it undergoes to fit between the two diameters. If it's really a PITA to remove the line, spring for the $15 in seals and be done with it. Personally, i'd atleast inspect the old seals.
How old are the current seals?