Guys I own a KLX 150s in South Bali (west-denpasar, just off seminyak). I am there only a few weeks per year so I'm trying to rent out the bike while I'm away. My friend who lives in denpasar rents out the bike for me when I'm away (and i JUST got back from bali).
If you want to rent a klx 150s in south Bali just give her a ring or a text at +6281805555319 (ketut).
If u are looking for something heavier to work with you should look in the streets for motocross(-like) shops. Often they have motorbikes in front of their store/garage with sometimes a for sale sign or no signs whatsoever. The reason I'm pointing this out is that you can probably just go there and ask them if you can rent something and for the right price I'll bet you they are interested.
In fact, I was last week in denpasar at a motocross garage/store and there was a big yamaha sitting outside, but no sign on it. Maybe if you are interested in that bike or perhaps they have something similar you should take a look there. I took a picture with my phone of the guy who worked there's businesscard and the adres is: jl Imam Bonjol no. 20 Denpasar.
I know there is also a motocross garage/shop near Ubud. on the road just south of Ubud, just south of the monkey forest...
If ur looking for klx150s for rent: contact above number or look in the streets for klx's with a for rent sign, but i found they were very hard to find.. (maybe because its high season and they were already rented out).
About tracks in Bali:
There is a motocross track near Ubud (south of Ubud) and there is a private owned one near the town of Negara in west-bali.
Furthermore you can do a little bit of fun offroading at the town of Serangan which is on a small island south of denpasar, where u are also surrounded by beautiful nature.. Its not a very exciting offroad trail or something, but I guess its pretty nice to just go for sometime and just enjoy.
It is here: https://maps.google.nl/maps/place?ft...KsOI8gPK3IGADw
anyway, regardless: I hope everybody has an awesome time riding in Bali. I find it to be incredible, especially if you're riding away from denpasars metropolitan area where it is much less busier traffic wise.. I'd suggest to head to west, east and north Bali. Just ride!
edit: I made a small website for renting out my kawa: http://motorbikerentalbali.weebly.com/index.html
there is also a motocrossshop here where u can take a look.. If there are bikes here u can just ask if they rent it out. Knowing how the indonesians bargain, I guess they will almost always start out with a ridiculous asking price, just so u know: Bargain! The adress is Jl ayani, Denpasar.