A stock 998 head (which is not a 12G295 Cooper one) won't go any better than your existing 850 head. Valve and port sizes are identical.

If you find a 1098 head (12G202) it has bigger intakes and ports than an 850, gives a few more HP on an 850 or 998. These were also used on the 997 Cooper and early Spridgets, so there are still quite a few about.
I'd deck it about 1.5mm before fitting, or your C/R will be lower than it is now.

Kevin G

Yes, it will fit an 850 but realize it will require a bunch of work to get it to go on-- new valve seats @ probably $8 each plus new valves and maybe keepers to match. I'll give you the old valves, springs, and keepers too, but you'll probably want to replace all of that to match your valves. When done, it's considered a "stage 3" or more head and would run about $600-$800 out of the box, but it still needs I'd say $200-400 retail worth of parts/work to get it to that point. If you can put in your own guides, ream them to size, install and cut your own seats and face it yourself, you'll end up with a nice head for cheap. If you have to pay full shop rates, it may not be worth it and just find a used head for the short term.

 Again, my email is in my profile. Drop me a note if you want pics or whatever.

--Spank 

Timmini

I have a 12G295 small bore cooper head. It needs new intake and exhaust seats installed (it had new seats installed and those have already been removed), and probably valve guides as well. No valves, just the core head. I'll sell it for $50 plus shipping. Think of it as a ported 998 head and uses 1.21 or 1.25 intake valves typically. They put them on Innocenti (Italian) 850 motors, too, for more umph.

--Spank

my email is in my profile if you're interested. 

Timmini