Yeah you get rental bikes there. You have to check the bike yourself to know whether it's in the proper condition or not. Mainly check break, clutch and carborator. And the price of the bike depends on the bike. Don't pay more than 1000 for classic 350.
Hey, Jalori Pass is a lovely strech offering picturesque vistas. There are quite few people I have seen you have been there on bike. You can easily find bikes on rent from Manali and Shimla. In Manali there are numerous bike rental shops near Mall Road towards Rohtang Pasa and in Old Manali.
Well I have had few friends who did Jalori Pass Cycling with YHAI - Youth Hostel Association of India. They will provide bicycles, food, accommodation etc. Not sure if they still have an active event. But you can check their website. http://www.yhaindia.org/national-level-program.php You can write an email to YHAi, even if they do not have an event, they are a helpful group and will give u necessary contacts