How big'ha engine can I put in THIS?

http://spokane.craigslist.org/had/4085772137.html

Scooter Store Going out of Business Sale! - $100 (Spokane Valley)



All prices negotiable, Everything must go! All makes and models, new and used power wheelchairs, scooters. Parts avaliable. Deep Deep discounts to move everything fast! Wheelchair vehicle lifts also.
Call for more info Ben 509-671-1xxx

Prices starting at $100 up to $500, really good deals, while supplies last.