2.75 X 18 is actually equivalent to modern-day 80/90 X 18, wherein 80mm is the width of the tyre, 90 is the sidewall height as a percentage of tyre width ( 90% of 80mm ) and 18 is the rim size in inches. 42 and P are the load index and speed rating respectively.
So, if you are looking at buying new tyres for your Karizma then look for 80/90 X18 size tyres. Speed rating and loading index of these tyres must be equivalent or higher than P and 42 respectively. You can go for tubeless tyres if you want to.