i'm trying to achieve the same thing you mentioned in your first post... a ta23 bonnet on a ta22.. and i think (in theory) that it can be done...
my thoughts are this...
grab both bonnets and split the frame from the skin, then restick the frame from your ta22 to the skin of the ta23, with some panel beating mates you can lip the edges around. now the tricky part, with the top end use some type of router, you can also use your old ta22 skin as a guide to router the top (near the window) part... you'll also need to smooth out the power bulge lines as they get close to your window assuming you want a smooth finish...
i've asked some panel beaters and boiler makers and they tell me it's totally possible, so yeah, sounds like a lot of trouble but if you want it and you got a boiler maker or panel beater buddy to help then it shouldn't be too hard...