1- Yes, back in the 1980's/90's you could purchase electric start kits and recoil pull starter kits for the KT100, but I wasn't able to find any currently available at the online kart shops.
2 - Yes, there's several different brands/models of pull start, 4-cycle engines you could install on your kart. You'd need a different motormount, but we could help you find one that would fit and bolt right on.
But one of the biggest Issues you'd have to deal with is: The output-shaft/clutch on these 4-cycle engines are located on the opposite side of your kT100. So one option would be to dissemble your axle and move your sprocket to the Inside of the axle bearings, just like the the kart in the pic below:
Does your axle have a keyway machined in to install a sprocket in that area? Some axles like yours do, and some don't...