There are a few possibilites. First, there's the freeware Maintenance and Income Tool (http://www.fsdeveloper.com/forum/threads/released-m-i-t-maintenance-and-income-tool.434351/)
This plugin can create random failures and also checks for damage (i.e. landing too hard). It's been a while since I've used it but I think you can configure the random failures rate as well as most other things. I think this may be just what you are looking for.
There are other tools available as well, such as Perfect Flight (payware), which can create random failures as part of it's features.
Last but not least, if you want a truly realistic simulation of failures in GA aircraft, there's no way around A2A's range of GA offerings. They have a full maintenance and failure simulation and are well worth the price. Also highly recommended is the Aerosoft DA-20 Katana 4X, which is every bit as sophisticated as the A2A addons.