lol i had a customer do this once
although they hadnt realized the mistake at first
i couldnt tell what fuel was in there siince it was a sewage treatment plant and the tank still had a strong gasoline odor
a few hours later the operator fessed up and said he filled it with diesel

if youre not a mechanic i would suggest calling one due to the dangers of working with petrol (gasoline gas etc)

change the spark plugs and fuel filter dont mess with the carb at all (yet) since it ran ok before (or if you got some skill drain and clean out the bowl)

if you have an h20 nissan engine the gap for the plugs is.028" -.031" (.711mm -.787mm)
and for the j15 its.031" (.787mm) -.035" (.889mm)

after changing the filter and plugs run it out of a clean gas container (not from the tank) and be careful of static discharge
leave the tank on the ground
get it running like that then
the gas tank (petrol tank) should have a plug in the bottom to completely drain it

if you still cant get it running send the carb out to have it checked or rebuilt

i dont think the diesel would present a problem in the engine itself the one i had above still runs fine

the compression isnt high enough in a petrol motor to ignite the diesel
diesel is explosive under high heat and high pressure
most diesels are in the 20 to 1 range where this motor is probably closer to 10 to 1

i used to work at a diesel fuel shop and ppl would fill diesels with gasoline by mistake all the time
those were ugly and expensive