Clutch slipping is when you push the pedal, your car revs, but does not accelerate. If you have that, then you need to replace the clutch disk, but no one replaces only clutch disk, so you should replace all clutch disk, pressure plate and bearing (the whole clutch kit). Gasoline engines do not stress the DMF that much so in your case when installing a new clutch kit, if you do not have problems currently with the DMF and when checked manually the clearance is fine, then no change is required. DMFs on gasoline engines tend to last more than ones in diesel engines, but have in mind that clutch slipping is stressing the DMF additionally so you better fix that as soon as possible.
Also converting gasoline engine to SMF is actually not that bad and you should not find that much difference, but have in mind that the torque will stress that otherwise fragile gearbox more, but it should handle it fine.