Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
If you have already drained the tank, and you are happy that, I would also flush the lines out and change the external fuel filter.
If it hasn't run for 6 months, your battery will need possible replacement.
You also may need a spray of carb cleaner or ether to allow it to start up again.
The last part of diesel should blend in ok if you have filled the tank with petrol now.
Do you have an air compressor?
If you can identify the fuel lines near the tank, one would be a feed line from the tank, one would be a return line from the engine to the tank.
That whole loop would be still full of diesel, if you can break the loop and blow the diesel out, reconnect the feed line, setup return line to a drain tin, run the pump to flush the last part, reconnect return line.
The spark plugs could be diesel fouled/wet, but some start ya bastard etc should help.
Hopefully after all of that the engine will start.
Yours is fuel injected isn't it?
If it is now out of your league, which this is for most, I would suggest you take it to a workshop for this part.
You have already done the hard part, unless it does have a drain bung in the tank.
If you can open the lines in multiple places you can blow the lines in any direction you like if you don't mind the diesel fuel mess, which you can clean up during or later.
Once you have the excess diesel out, reassemble the lines, and it should start.
How did you go with your problem?
I hope my advice has been of assistance to you.