Ancient adhesive on plastic key tops

[ReggaePass@aol.com]

Dear List,

Well, actually, the adhesive is not ancient, it's just been on there for a 
LONG time.  Someone had the great idea of I.D.ing the naturals in the 
centermost octave of this 25 year old Cable Nelson console with pieces of 
masking tape.  That was a decade or two ago, much of that with the piano 
stored in a garage (where it gets really hot in the summer).  Now, if you 
manage to get the tape off, residual adhesive stubbornly remains.  

So...how can the tape and adhesive be removed without harming the keytop?  
Would a light application of acetone promptly cleaned up do it, or will the 
plastic react right away?  If solvents are not an option, what would be the 
best way to nondestructively scrape away the unwanted stuff?

Thanks and praises,

Alan Eder, R.P.T.