Welcome back.

The only clearance issues you might find with the A15 is the stock air filter to bonnet, but a lower induction system like your RFI will fix that little problem.

At worst, some Cortina engine mount rubbers & some flat steel, & you're done.

The 1000 radiator is, .... as has been said, larger than the 1200 & works well. You will find that with more powerfull engines that you use high power settings less often & for shorter periods when driving normally, so the engines heat output is not much different from an overworked stocker. I think that your radiator will be fine.

I used an unmodified stock radiator in my 1000 sedan with the GX engine for many many years & it never overheated, ...... ever.

For street use, I suspect that 150 hp is a bit ambitious & un-necessary. My GX must have been only a little over stock [about 90 at the fly] & I loved it. If you can get somewhere between 100 to 110 hp you will be amazed just how well it runs. The additional torque resulting from the extra 1/2" stroke of the A15 will transform the driving feel & make you glad you made the switch.
If you tow anything [I did] then that will feel much better too.