Tom - assuming “strop” = thickest line on the bar, that runs from chicken or harness loop through the centre hole in the bar up to the pulley & swivel, back through the bar and to the chicken loop - a.k.a. the trim line
Either order a replacement trim line from your OR dealer and it will come cut to length and with the appropriate fitting on its end for connecting a leash & also the threading “tongue” described below…
Or if you’re a do-it-yourselfer, replace it with a 5 mm or 3/16 inch diameter single braid Dyneema or Spectra line ( Samson rope ) usually available at a marine supply store.
To make it easy to feed the end of the rope through all of the places it has to go, cover about 1 inch end of the rope with a long & narrow ( use about a 3 inch x 1/2 inch ) “tongue” or leader of sticky-back sailcloth. Electrical tape or duct tape might work too.