Been there, done that. My condolences.
When my 100-footer went down exactly the same way,
I sawed off the remnants of the original legs
about an inch above the original base (it was a
full-size section set in the concrete), and
stuck pieces of rebar down into the holes as the
first part of a rebar cage extending about 6 inches above the old base. I then
built a form
about 9 inches tall, and poured concrete on top
of the old base. Just before the concrete set, I inserted a standard Rohn pier
pin, and went with
the flat baseplate. Yeah, Steve, it requires temporary guying when the tower
goes up, but
I feel more comfortable letting the base plate rotate around a bit than
bolting the baseplate and wondering if I got it really straight and whether
the bolts are stressed and at risk for faster corrosion (stressed metal
corrodes faster, I've been told).
My other tower is also on a flat base with a pier pin, sitting on top of just
enough concrete to level the top of the huge rock that was sitting where I
wanted to dig the tower base. I figured there was no sense digging up a huge
rock just to make a hole to install a man-made concrete "rock", so I rented a
rotary hammer and a 1-inch rock drill bit and made a hole in the top for the
pier pin, backfilled it with hydraulic cement, and it's been up for 12 years.
73, and good luck,