As VICKY has already replied, both engines look same from outside but inside they are different. I did not believe that also but recently I got mine full serviced. Air filter is bigger, fuel lines are bigger, carburetor has more jets in it and bikes ground clearance is low. Which is good for off the line and more speed. Oh and 1 more thing 5th gear, if you are going to race some, dont give the bike 5th gear until it reaches speed of 100km/h. 5th gear slows the bike down. Now you brought in physics, so my friend if you read physics, that means you should know about aerodynamics. They always help you to get up to speed, helps breaking and acceleration. This is where Deluxe as advantage. It pumps more fuel in also to give more speed. Cruiser Tyre doesnt give you grip but it helps you in speeding as the name shows.
If you can visit a good mechanic go to him and ask him the difference between Deluxe and 125.