Mate, augustvis peak monsoon across India, not fun.
We live up in the north, about 2 days ride from manali in a town called Mussoorie. If I were you I would fly into Dehradun, then hire a bike there or get a bus to manali and hire there. I can put you in touch with a reliable bloke who helps me out with parts and advice, he's been running Leh tours since the 90s.
You can also fly into Ladakh as well.
Personally I would skip on riding on the plains.. It's hot, wet and terrifyingly manic on the roads, not much fun. Fly up here to the mountains and avoid the mayhem of the lower altitudes and ride into the rain shadow!
And we will take you for a
if you pass through our way