That new top tube makes this frame look 100% radder! Love it. I do like the original fork better though, especially if it's a mountain bike. Unicrowns aren't typically the most appealing, but in this case I think it flows better with the frame than the other fork. Just my 0.2!
Looking forward to seeing where this goes, well done so far!
Use the spray adhesive to attach the leather to the foam leaving some cloth hanging down from the edge. Wrap this overhang under the seat edge then drill holes all the way around pan. Then just run some lacing aroun the edge,
Here is how I did a seat but you will find lots of other samples people have done witha simple site search.