I retained the tin to shield the epoxy filler from being blasted when I use the mower. If you are after full on restoration with no plans to actually use the mower, then you could get away with removing the backing. One idea might be to use gaffer tape or maybe even masking tape will make enough of a surface to mount the filling material onto. It will be tricky to achieve a flawless bridge over the hole on both sides. I would try doing the outside and then the inner in separate stages.
It would be ideal to find a skilled lead filler if you are mega fastidious about achieving an authentic as possible restoration.
I personally think bog or epoxy filler will be perfectly practical and then just paint it over.

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!