Hey, that's my bike! haha. Well, it WAS my bike. This thread is 5 years old, and many things have changed during that time.
As I recall, the bike's handling was very nimble. It was simple, relatively lightweight, and fun to throw around.
My plan was to keep the frame intact, but braze in a second top tube and second set of seat stays along with a seat tube extension to stiffen the seat area. Similar to how some F1 bmx bikes were designed. In the end, I disassembled the bike and stored the frame and other parts for years, only to have recycled them about a month or two ago.
Here's what I like about the concept: compact frame, bottom bracket in "correct location" for 20"wheelset, if you get a bmx frame with a long enough top tube (~21") the bike is OK for adult size riders.
The challenges: the seatpost (as already discussed), need a riser for the handlebars, rear dropout spacing not correct for geared wheel.
I say if you can find a geared 20" mountain bike with a 21" or longer top tube, you might have something. Similar to the F1 bmx bikes that marxmini is using.