By the looks of it.....because you are in cat 3 you are in the up to 1300cc class. So anything from Hilman Imps, to Mini's, to 1300 escorts i reckon
If you can find an up to 1000cc class you will be up against Imps which are quick!!!
Check out the HRCR website for the championships they run and the regulations attached to them. There are of course other championships and local clubs who run historic stuff too.
http://www.hrcr.co.uk/
In my opinion, unless massively tuned, a 998 will be seriously out gunned. But if you drive it like you stole it and have a laugh doing it then the best of luck to ya...i'll be rooting for you anyway.
Don't dismiss the HRCR Clubman championship though. It may well be daytime and on open roads with tricky regularity sections, but the special tests are fantastic!!! I've driven on the Tour of Cheshire this year (won my class) and the tests where varied and great fun (disussed runway, farm tracks, flooded tile works etc etc). Then a few weeks back i navigated on my first event (again won my class) which was the Ross Traders in south wales. This event was mega!!!! The regularity stuff was a real brain ache (but thats the nav's problem!) but the tests were great. The first two where on forest tracks and again rough and exactly like proper stages, just with coned chicanes to slow you down....they use this forest on the Wyedean rally i think. Then we did several runs around Chepstow race courses gravel infield tracks...again awesome fun!!!!
This isn't a you must do this rant....but they are very good value for money and you get a full days sport without needing to mega prep your car. Also, your 998 won't be out gunned quite so much as alot of the points are won or lost on the regularities. My plan has always been to prep my car for stages so i can do that in the future if funds allow, but compete in the clubman championship as its great fun, social and cheap (compared to stage stuff!)
Anyhoo....check these photos out and see what you think.....(i'm in the orange Triumph Dolly Sprint)
http://www.daisyroot...2013/index.html
Also...where abouts are you based?