Advise the seller of your intention to seek legal advice. Its a small claims dispute.
Consult with a lawyer after about a week (time for seller to respond). This is what they are trained to deal with - they will cut through any confusion and advise appropriately.
For example - it will be noted that replacements are acceptable and offered but due to circumstances its simply a waiting game.
The lawyer will ask you lots of questions, so *be prepared* with a detailed timeline amongst other relevant matters and documents. The technical things you will explain to them in human words.
You paid for what is known in the car industry as a lemon. Happens sometimes, you know.