I have an older plum tree I really like. It’s been moved and stressed and not really in an ideal location. It does have healthy growth that I would like to graft some onto the proper rootstock. How do I determine what the best rootstock for it is. I do not know the variety. It flowers very early, 3/7 this spring. It is semi dwarf and produces squash ball sized purple fruit. Not so much anymore. Are there particular sources people can recommend for rootstock and possibly even do the graft as I have never grafted to rootstock before. Thanks all.