What is the gas/oil ratio for a mantis rototiller?

If you have proper timing, compression, gas and a spark, the engine should fire/run.
Have you tried a spritz of starter fluid to see if it fires ?
The flywheel could have sheared the key and affected timing.
About the last thing I can think of is valve adjustment, specifically the intake valve, although I think it would be difficult to pull the crank rope if that's the problem.

Hi ANON, in my mantis i use a ratio of 40:1 the one being oil.

First check that your Mantis needs an oil/gas mixture, most of the newer Mantis tillers have four-stroke motors meaning they have oil in the crankcase and use straight gas. If your Mantis needs oil mixed in the gas use a 50:1 ratio. That's usually 2.6 ounces of two-cycle oil (it's sold in bottles that size) to one gallon of gas but read the oil bottle to get the proper mix ratio.

The ratio is 50:1. 2.6 ounces of 2-cycle engine oil to 1 gallon of gasoline. Convenient, pre-measured bottles of premium 2-cycle engine oil are available via our on-line shopping on link below.

http://mantis.com/store/viewitem.asp?idProduct=1478

http://mantis.com/store/viewitem.asp?idProduct=923

Good Luck and please rate 4 thumbs up for the free answer.

You're okay the white smoke is the oil burning off the muffler. In time it will stop, just park it somewhere and let it run until it's done. If the problem persists check your air filter and make sure it's not stained with oil, if so you can wash the foam part with soap and water but the paper will need to be replaced. (DO not put foam back on unit until dry.) The oil level should be up to the bottom lip of the dipstick access when the engine is evenly horizontal.

IT is 200ml to 5 ltrs of unleded petrol

The 2 cycle needs oil and gas mixed while the 4 cycle needs the oil changed every 10 hours of use. 2 cycle is 28:1

I would not run it any any less that  50/1 ratio.

jj505 has good and bad information.
Honda GX25 is a 4 stroke motor, with oil lubricated crankcase.
Oil drains out the filler.
Oil and gas do not mix on this four stroke.
The PDF below is the Honda manual.
http://www.smallenginewarehouse.com/images/manuals/GX25-35.pdf