I have not found any solvent that will dissolve fibreglas resin.
You have to use mechanical abrasion to remove old resin bedding material, as new bedding resin will not adhere to it. I use freshly sharpened old wood chisels and gouges on the parts that won't show on the assembled rifle, and then use rotary file type cutters with a Dremel tool to work up to the stock/barrel/receiver line. The gouging and cutting only needs to allow the new bedding to be a minimum of 1/16" to 1/8" thickness.
HTH
John