Should I replace my Forester with the new model?

After lots of research I bought a new Subaru Forester to serve me in my job as a sales rep and it has now done 306,000km trouble-free except for a slow oil leak in the head gasket, first detected at 220,000km and which I was advised could be monitored before anything was done about it. I am now told the gasket needs to be replaced but seeing my car has an aluminium block there is a 50:50 chance that removing the bolts will strip the thread. If all goes well it will cost me $2000, but if I need a new engine it will cost at least $6500, which is quite silly to spend on a car with so many kilometres. The problem is I will be 68 this year and will retire within the next two or three years so will lose my car allowance, which has paid for the car to date. Should I take the chance and spend $2000, hoping all will be OK, or bite the bullet and replace the car? If I knew that I will work for five more years I would happily buy another new Forester or perhaps a Kia Sportage.