I picked my engine decals up on ebay. Here's a link to two of them...the oil fill and Easy Spin decals that would have been on the original 10 horse Briggs. The 'Allis Spec' decals came with my full tractor decal set. I got mine from Wells Implement, but they can be had from maplehunterdecalstexas.com or from clickitandstickit.com as well. I've used decals from all three sources and find their quality equal.
With some looking around, Briggs decal makes can be found...I actually still need the large Briggs shroud decal from the carb side of the engine. Haven't really looked for it...
This project was absolutely beautiful. I have a Big Ten that was my Grandfathers before he passed. My Grandmother gave it to me after that. I love the tractor. So many memories from my childhood. Some day I will be restoring it and I hope it comes out as great as this. I will definitely use your project as a reference.