Well actually the repaired one at least gives you a template to follow with the new one.
Re repairing it … not sure what others would suggest here but my opinion would be - but it’s your own choice. However I get the dilemma you face here.
PVC bonds very well to fibreglass given the right prep.
option 1
Take a low grit sandpaper and really rough up the whole inside face of the PVC backplate and broken piece. I’d magic tape the crack back together FROM THE OUTSIDE as it’s low tax, and use it again to hold the missing piece in place AGAIN FROM THE OUSIDE (Painted side)
Basically you want to hold the break together and hold the broken off part in place while you repair it from the inside. PVC bond may be an alternative to tack it in place.
Also block up using plasticine/clay etc ..the slots for the straps on the or resin will likely leak through and all down the outside.
ID then suggest GENTLY paint on a layer of fibreglass resin onto the whole roughed up inside face of the plate especially the crack and around the broken part. When that’s cured then use a second layer of resin and pieces of fibreglass tissue (not the heavy mat) again on the whole of the inside face. **Watch a few you tube videos on that**
make extra sure the strips of tissue/mat cover the crack and broken off part edges.
Note - it’s horribly messy stuff so have the backplate stable and held or get someone to hold it as you works
assuming that goes ok and the resin repair is still fairly thin I’d then do AT LEAST one further layer with the tissue BUT covering a much larger area of the inside to create a strong inner shell and finish with another layer of resin …
Fibreglass resin generate a lot of heat as they cure so that’s why id suggest thin layers at a lime building up a reasonable inner shell.
Also Also … leave it to fully cure for a day or two before you do any more.
that should then HOPEFULLY leave you with only the minor cracks to fix up from the outside. Fine model filler and light sanding will do that and then a touch up of paint. I doubt you’ll fully disguise it but if you can get he paint spec used by the seller a reasonable repair should be feasible.
here a link I found as well that may help I’ve not read it I n detail so it may have better advice.
best