Not sure if its a big deal with the little 49cc, but when doing a motor plate on a bicycle I like to weld a support bar across the frame first, then weld the plate to that.
Reason being, its hard to get a strong structural weld with a thick plate to the very thin bicycle tubing since a hot enough arc to weld to the plate will burn through or fatigue the thin bicycle tubing...and a cool enough tip to not damage the tubing won't get much penetration on a thick plate! Not to mention it will help keep it from flexing and help tie the front and rear of the frame together.
When I stretched my beach cruiser and put a motor plate on it, I notched a 1" square tube with a hole saw to weld in across the seat post to down tube, then welded the plate on top of that(along with the normal weld to the frame tubes).