It is really cheap to implement -- you can use a shift-xor random bit generator to get the blend factor of ½ that they mention. The whole thing was about 10 8086 instructions, as far as I recall (it's been twenty years since I played with it).
Here's a paper with more extensions to the Karplus-Strong algorithm (+ analyses thereof):
(I haven't tried it out so I don't know how good it sounds.)
There's also a master's (or phd?) thesis from DIKU (Datalogisk Institut ved Københavns Universitet) from 15-20 years back where a guy "rewired" sounds played on one instrument to sound like they were played on other instruments. I saw his presentation/demonstration back then, and it sounded pretty okay. It's in English and should be available somewhere on DIKU's website. Darned if I can recall the title or the author, though.
there's also 3 good basic textbooks on synthesis/sound design by Rick Snoman (Dance Music Manual), Martin Russ and Brian Shepherd that have specific recipes ("detune a saw and square by 3 cents...)
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and if you don't have a synth, ask around for somebody that has Logic Pro X (several good synth models included) or Ableton with VST or AU's installed (you have to pay e.g. $100 for Analog unless you have Ableton Suite).
A good book that provides a quite detailed overview of some physical modeling algorithms (and tons of other synthesis algorithms) is The Computer Music Tutorial by C. Roads. With regards to Karplus-Strong for example, it provided an in-depth enough explanation of the algorithm for for me to base an implementation solely on it. There is another description of a flute instrument model, using noise, filters and delay lines.
