There is also the Organelle from Critter And Guitari which runs PureData on arch Linux on the raspberry pi. It is also a synth/sampler (though sequencer-based instead of "tape") with a similar form factor to the OP-1.
There's a really cool project for the Organelle called Orac[1] that turns it into a super cool modular synthesizer. It's also available for Raspberry Pi and Bela Salt
There is also the Bela cape for the beaglebone which, along with a special os for the beaglebone, gives you a very low-latency platform for synthesis, programmable with Supercollider or puredata, and lots of analog and digital io.