How exactly would you do this? The forester is unibody, you would have to fabricate all the suspension, control arm, diff mounts, engine cradle etc from scratch. I don't think you would end up with anything even close to driveable. Also I doubt you could insure, register, MOT etc it afterward!
Running RWD or 4WD? You'll never use Cortina front suspension with 4WD, you'll probably also struggle to get the flat 4 engine between the narrow Cortina inner wings (Narrow due to the width taken up by the original double wishbone front suspension)....so you'll need to widen the engine bay, new longitudinals and new front suspension I suspect.
Sounds like more trouble than it's worth really. Am going to investigate my second option. With it's far larger engine bay, A mk4 zodiac might be worth while. But am thinking that the engine may struggle with such a large heavy car. The fossie is going to end up a project of some sorts. I'm just considering options for the new year. One other is turning it into a pickup.
Get a FWD transverse bodyshell, the bay will be wide enough, subaru running gear has already been put under a Focus with just some minor trans tunnel mods, fabricated mounts at the rear, Subaru front X-member on fabricated mounts and using mixed and matched Subaru and ford front end bits.
That or use an Audi shell where the engine/trans layout is similar!