I get a fair amount of my stuff I need right now from joanns but since they are my closest fabric store and still a 2 hour round trip I tend to do more internet ordering. All of my sewing is on a singer model 66. I do have some different feet for it stuff like a walking foot is out of the question. I've done my fair share of piping, welting, and zippers without the correct foot. I personally don't buy actual scrim foam. I buy plain foam and glue material on one side. The material is normally bed sheets I pick up for next to nothing at a yard sale or goodwill. For that I use the cheapest spray glue I can find. I think it's elmers brand from wal-mart right now. For gluing the upholstery work pony up and get 3m super 90 (super 77 lets go in the summer heat around here). You normally only glue stuff like motorcycle seats with a metal pan though. Staples are the attachment of choice on stuff with a wood base.
A mini bike seat is a good project to start with. I'm in no way trying to take away from the seat makers or their works of art but most are pretty simple shape and design making an easy project. I jumped in over my head doing my own car and truck interiors and had a ton of expensive redos before I finally learned. Real pain learning stitch shrinkage doing pleats or diamond tuck after you have done a whole bench seat top to match a bottom that does anything but match lol.
If that is your first seat you done a really good job on it. Way better than I done on my first, I didn't even take a picture of it. Before you cut it apart I'll suggest adding more foam under it to fill it out. It doesn't look overly large and you might just be able to stuff it and fix that. A bit more cushion rarely hurts. Now if you were 4-5 inches too big yeah you might have to redo it.