Check it for compression, and spark ( spin it over with the plug removed, making sure that the plug is grounded to the engine)!

Block off all the holes into the engine, and give it a good clean with a de-greaser, ( maybe better to remove the coil)

If it's a relatively new engine, it probably won't be necessary to disassemble it.

Next step is to remove the cooling vanes from the flywheel and fit an extended prop hub.
Remove the flywheel by loosening the nut holding it to the crank, ( just a turn or so) and holding the flywheel, give the nut a good hard smack with a copper hammer or similar, and it should come loose.
If it doesn't, take it to the mower shop, and they'll remove it in no time.

You can then break off the flywheel vanes with a small ajustable spanner or similar, and clean them up with a rotary grinder or even a file.
Always remove equal amounts of metal from each side the flywheel, so as not to upset the balance.
The prop hub will have to be made on a lathe, and if you're stuck, I may be able to help there, - just ask.

With the exhaust at the back, probably some sort of spacer or "standoffs" will be required, also possibly some exhaust modifications will be needed, but that can come later.

Dave (Zagnut) I believe is quite experienced in conversions, and machining, and will no doubt be able to offer heaps of good suggestions, but I am just down the road if you require a little help here and there.