Motorcycle Questions? Ask a Mechanic.
Tough to know based on the information given. A 1.8 starter is pretty strong and should do the trick. Let's see if we can try to narrow it down. I'm going to need some more information.
Do you have compression releases?
If not, do you know your cylinder pressure in each cylinder?
Do you know what cam, lifters, and pushrods you have?
Have you tried removing the plugs and turning the engine over manually in 5th gear to insure everything is free to turn easily?