With my luck I wouldn't be able to find a matching shade of muslin on the planet. I thought I might try a kona of lighter or darker green or ditto for lighter and darker red. "Ever heard or seen shadow quilting?" I saw it a while back by one of the Wells (mother or daughter I can't remember). You lay out a background fabric, set the elements, set the elements on top, lay a sheer fabric on top and then quilt around the elements (pattern or little cut outs or blocks) and that's all I remember. I guess you finish it like a normal quilt.
I think this would preserve your quilt, mute differences between the old muslin and the new muslin and let you isnore any differences in shapes of old blocks.
Books on shadow quilting are still in print, even buy used on Amazon. Or just check some of the internet sites.
Jois