There was a rotary-powered MGB at the Moss Britfest (here in NJ) a few years back. It was a light color and had a lowered, "Speester-style" windshield. The owner told me it was a complicated job because, even though the engine is small, it's a "funny shape".
And I wouldn't exactly call a 13B in a Spridget "popular". I've seen two and one was a tube frame. The other (the yellow car that was featured in Grassroots) took a long time to get right I was told.
I saw a Mallock with a 13B yesterday (at our TT at Pocono raceway). The engine had to be offset quite a bit compared to the regular BMC engine due to the odd shape of the rotary.
Probably the nicest swap that used to be popular in Spridgets was the old 1600 Kent engine and 4 speed trans (from Cortinas or Pintos). These days, the British "K" motor with the Ford Sierra 5 speed is a somewhat popular swap.
A good, but not easy swap for MGBs is an 1800 Miata engine and trans.
When all is said and done, my favourite engine in the MGB is a nice, "worked" BMC 1800.
And I think Spridgets are best with a nice 1275.
My tuppence.