If you have an old Rotax engine with zero upgrades. Its not worth it to upgrade in my opinion.
Old black coil change to new style green coil. $450.00
Clutch conversion kit with balance gears ect. $500
2009 or newer cylinder $600-700
New style air box around $150-175
With a new cylinder comes the need/cost of a new piston kit $150. Plus the cost of dealer/service center for labor. Clutch being a newbie done by service center. Coil and airbox you could change out.
Rebuilds are based on hours of use on engine. Rotax Top end about every 20-25 hours and bottom rebuild 40-50 hours of use.