Hi
Sorry for delay, just read your post. I built a trials Bantam recently and came across exactly the issues you are asking about. You can take a look at the blog I wrote about it here http://myweb.tiscali.co.uk/f1racing/bantam.htm
In short, you just need a Tiger Cub swinging arm (try Greystoke Engineering) as these are a bit wider,but are a straight fit for the Bantam frame.
As for forks, virtually any can be made to fit. If look at the blog I wrote, you can see how straightforward it is to convert almost any. With the plethora of off roaders now finding their way into the breakers yards, you will be spoiled for choice. If you get one complete with a 21" front wheel most of the problem will be solved for you.