It all depends how far down the line you are with the restoration. If you've stripped the body back to bare metal, dealt with all the rust and then had the body painted, then I'd say it's worth speaking to a few Jaguar specialists to see if you can source a replacement engine. However, if you were still in the early stages of removing items and assessing the project, then it's a good time to take stock. Very few cars will be worth the amount of time and money you've spent restoring them - but sometimes that doesn't matter. If the car is special to you, and you enjoy restoration work, then press on. Maybe investigate an uprated engine - try and find the positive in the situation. Old cars are a labour of love, after all. Good luck with whatever you decide to do.