I got a code practice oscillator for sale; $12 but it mostly works with a straight key, and has build-in decoder that will display the letter you keyed up; its the one shown here https://www.spikenzielabs.com/Catalog/spikenzielabs/the-telegraph-decoder-kit sans the straight key; and you can change the nitty gritty detail on how long is a dit/dah by using a FTDI breakout to reprogram the microcontroller (which runs on Arduino sketch, code available from manufacturer)

The ATMEGA328P will worth that price, for your reference.