Have a search Ive done a few [osts on the subject the 100 is quite a bit uprated over the 85 as jeramy said new pistons, rings, turbo, fueling, oil cooler. etc
There 2 tunning method though
1 - Remap ie superchip which is expensive but the best way round it (cost abotu £450 I think although members get a discount)
2 - Tunning box connects in a few minutes and cen be removed easly for warrently work or selling it one. Cost about £250 to 500 but you coulds sell one on for £150 minimum.
Gains wont be that much different the remap will probably get you another 5% more power but thats only a might not for definate. You will get better power delivery though but it will cost you more.
Knowing the price for a new enigine (over £3k) Id go for a tunning box just so Renault wont know start trying to back out of warently work.
You can tell what engine you have by the side badges the 65 has white letters the 80 (some call it 85 as thats what it make in real life) has a coloured I (think its blue). The 100 has 2 letter coloured in (or is it three?)
You cannot just put a 100dci ecu in there. Well you could but please invite me to watch and video the engine when you do.
You could remnap a 85 to 105~110 at superchip or tunning box to 100~105hp.