You will be very lucky if you can actually get nuts onto the bolts , there is very little space.
Add to the the upper surface is not machined flat for the nut to press against.
So self tappers are the only way to go.
If the engine did not come with them get 5.
One to cut all the threads with and 4 to mount the engine.
Thread the holes before you fit the engine to the mower.
I use an air wrench , powered screwdriver or electric drill depending upon where I am doing the job.
I have a mounting bolt that has been turned down slightly smaller ( running it againts a grinding wheel ) that I use to pre thread the holes.
Thus the holes ar threaded a just a little smaller so a drill / driver will wizz them up no problems and the small bit of deeper thread they cut will keep them in there nice & tight.
You really can not use a standard Unified thread tap as the bolts are not unified pitch and the tap will cu the threads too deep so the bolts will tend to come loose in use.