If you accept a repair, then technically you have accepted the car and then cannot reject it later on.
Also as you have done work on the car yourself, he doesnt have to accept it back from you as its not in the condition in which he sold it to you. You have to return it in its original condition.
You have a short period of time in which to REJECT the car,
Close Alan...
If the repair isn't done correctly, you can still reject it.
The car doesn't have to be the same as it was when sold. The dealer can request that a refund is minus any wear and tear costs (prevents you from buying a car, driving to the moon and then returning it). As his new parts were serviceable parts, there is no problem with that.
He still has 6 months to technically 'return' the car and reverse burden of proof still applies. The law (did) state a 'reasonable' time window in which a rejection could be granted (often 4 weeks). But Given the lack of a head gasket repair, even when quoted/invoiced, then a court would no doubt agree that a rejection at this point would be fair - at least I have seen this done. In order to subsantiate his claim an independent report would be conducted that could add weight to this.
But... All this starts with a formal notice served to the dealer, which hasn't been done, nor does anyone think it will be!