I've plugged old beams on several projects and I like to use old tenon tips. I'll trim them off salvaged beams and save them for this use. As far as strength goes , don't rely on an old beams for support structurally ! They should be used as a decorative feature and be required to carry there own load only. Most old timbers are stronger than new sawn ones , but mortices and notches can obviously weaken the timber. When in doupt over build. There are also several methods for re-iforcing a spanning timber from above(if head room allows) to eliminate posts.