i have a 850 that i am about to rebuild as well. Biggest problem you will have is getting rings and pistons. especially if you need to bore it out. If you are going to get a few blocks, use the one that does not need a rebore so you can stick to the original pistons, Karcraft still has standard rings available. Otherwise you might have to creative and start looking at other pistons.
biggest help for torque will be extractor exhaust, which you can get complete for a few hundred bucks. then if you get a head like a 12g202, the flow in the ports will be freed up immensly, but you have to skim it to get the compression ratio back. 12g295 head is probably overkill for 850, but it all depends on what you want. Twin SUs will only be worth it if you get the head and cam changed IMO, but are good for bling factor. Apparently the 850 sports had twin 1" sus, and extractors, normal cam and claimed 40hp. That is almost 25% increase.

You could probably still get that with a single 1.25" su.
the previous owner of my 850 claimed that the cam was reground to "997 cooper" spec. When i start to pull it apart i'll measure the thing to see if that is correct. The rings were absolutey rooted, so i can't comment on how much improvement it made over standard.
The vizard book has some great stuff about modifying the 850, but is typical of the attitude that still prevails -i.e. why waste your time with an 850 engine when you can modify a 998, 1098 or 1275 ... as a consequence the supply of parts for 850s is almost non existent. Having said that he gives all the details on what is possible. The biggest limitation to a modified 850 is the pistons. He reckons 5500 rpm is about it for standard pistons (from memory). the cranks are not as strong as other engine sizes either so that is another limitation.
good luck with trying something different tho
michael
_________________
the world is a book, and those who do not travel read only a page
66 Mini Minor sponsored by
http://www.lifeonthehedge.com.au/ The Dog Harness Specialists
It was a pleasure ausmini. I'll miss all you misfits and reprobates
