Most cheap cassette players I've messed with over they years have had an adjustment pot either in or close by the motor, so you're probably half way there.
do you believe that would be enough of an adjustment to get that same effect as I'm after?
thanks
Maybe. It's probably 30 years since I last had my head in a cassette deck. All my open reel machines have synchronous capstan motors, so without a vfd that would be difficult. The cassette motors I dismantled all and little adjustable speed control boards built into them.
Buy one and find out. You'll probably have more trouble with tape tension and the associated mechanism if you try to make rapid speed changes, but there's only one way to find out.
Funny, I was re-watching a docco on Sgt Peppers on the plane tonight and they were discussing the invention of "Vari-speed" by George Martin and his engineers. Man, that was 50 years ago! Time flies.