welll,,,, here`s a can of worms opening
sooooo many of you (nearly all of you) have suggested the 1100,,, here`s my spanner thrown in then
I reckon that if you could afford a 1275 then just do it,,, but yes, you`ll want/need disc to help stop crashing
but,,, in my opinion, the 998 engine is such a smooth little thing, they`re a very very nice little sweet donk,,, you can over-bore them quite a bit, making up the difference in capacity to the 1100,,, but obviously not as much as opening up an 1100 with it`s longer stroke, so still doing the same thing to an 1100 as you would with a 998 won`t make the 998 any better,,, so in the Torque dept, the 1100 wins
but in the reliability dept, i reckon the 998 wins,,, & to give you an idea to mull over-->
i have (in my short life time) pulled apart probably more mini engines than i reckon just about anyone else on this forum & in all that i`ve seen more stuffed 1100 cranks than 998s
soooooo,,,, for longevity/reliability, i`d go for an over-bored 998
but i`m Massively biased towards the 1275 (or more accurately the 1400cc & 1500cc) engines that i`d have to still suggest that if you can afford a 1275 & discs,,,then do it,,, they are stronger, they are MUCH more torque y & are not high maintenance at all when built properly