I would keep the old motor in storage(in case you want to sell it as an original 850 in future) and fit something bigger.
How much bigger depends what $$$ you want to spend. You could build a 998, but a 1098 will cost the same and have more torque for road use. It uses the same block.
You will probably also want another gearbox to fit a 1098, if yours still has the original casing. It is possible to swing a 1098 crank in there with a bit of surgery, but the 850 box is old and probably worn. New parts are no longer easy to get.
If you go for a 1275 motor, you WILL need another gearbox. I would recommend a Mini-K, MK2 S, or rod change as they are all stronger and have synchro on 1st gear too. A 1275 will bolt straight on and crank etc will swing OK.
Also with a 1275 or bigger, most states insist you fit disc brakes. Probably a good idea anyway if you're increasing the power, 850 front brakes (if original) were pretty poor.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.