From investigating AGA ovens, in theory there is no problem in kashering them.
In practice it's gonna be quite complex. They have a ton of areas, and they are massive with a lot of thermal mass.
In order to kasher them, you have to heat them up quite hot, but because of the thermal mass it's going to take a huge amount of energy and a very large flame. It may be impossible to do it with a regular torch.
For a final answer you should ask your Rabbi about it - but I strongly recommend that you insist he see the oven in person, since it's not a typical oven.
Also, as a practical note AGA ovens are EXTREMELY energy inefficient. Very wasteful, I would not recommend them.