Can I use motorcycle engine in aircraft
You will need a fixed landing gear and a braced wing to keep your mass at your target of 40 kg. Expect a maximum L/D of not more than 10, maybe even 8.

You say that the flight mass will be 100 kg. The weight will be 980 N. 1/10 of that is 98 N. To arrive at a power from that will need a few more heroic assumptions. Your wing loading is probably 12 to 15 kg/m² and your lift coefficient around 1.0. This equates to a flight speed of 15.5 m/s (= 56 km/h). I assume a propeller efficiency of 70%, so your power for straight flight will be 2.2 kW.

For decent take-off performance, add 1.5 m/s climb capability at an air density of 1.1 kg/m³. The extra power to climb with those 1.5 m/s at 15.5 m/s and 70% efficiency is another 2.2 kW.

The sum is 4.4 kW which is 5.9 HP. Running the engine at this power level means 54% of maximum power. Make sure you have a gearbox which reduces prop RPM to not more than 2000 in order to make those 70% realistic. The wing will need to have about 8 m² area and the tail another 1.6 m². Will that all fit within your 40 kg?

I expect that 80 kg is a more realistic empty mass. Now the wing will measure 11 m² and the power demand climbs to 3 kW for level flight and an extra 3 kW for climb. Total power will be 8 HP or 75% of maximum power. Ask yourself how long the engine does this and how big your radiator needs to be in order to avoid overheating.