I think I'm going to take a few pages out of your book for my own beach cruiser! Hooray for trix
Actually my biggest thing when building a motored cruiser was that coaster brake. I did not want one at all. I have never had good experiences with them on terrain that's not perfectly flat, and the term 'high quality coaster hub' is an oxymoron. This is to sayy nothing of the horror stories out there from motored and non-motored cyclists.
Now, to my knowledge there's no actually -good- way of converting coaster to freewheel without replacing the hub outright. Long story short the rear wheel was pretty much the hardest thing for me to find what I wanted.
Having mused on the coaster brake, I'd like to comment on your chain idler.
Very nice. I might have to inaccurately reproduce that design because the stock idler on my bike is going to get me killed some day.