Engineering business organizations sometime have consulting contract templates, e.g. CELSOC. However, depending upon your need, you shouod always have a lawyer review any contract you plan to use.
Before you use 'any old contract' you need to decide what sort of 'maintenance' you want done. If you can define the maintenance in terms of tasks, quality and frequency, then you would use a fixed price contract; If you can't define the maintenance task in these terms, before you enter into a contract, then you might want to consider a reimbursement contract. In reality you will most likely have a fixed price element for those items that you can define in terms of tasks, quality and frequency and reimbursable elements for those items that you can only define in terms of type of resource required, as and when called for. But, I certainly wouldn't be considering the type of contract until I understood how and when the work would be defined. Eg if you wanted to replace all the light bulbs in your building every six months, then you would have a fixed price contract, but if you only wanted to replace the light bulbs when they stopped working then you would use a reimbursement contract. This example, hopefully, indicates why it's important to select the right type of contract for you.
If you go down the reimbursement route, you will need to define the type of labour that you think you will need, for the tasks that you expect to arise. Reimbursable costs will be made up of costs for labour, materials, overheads and profit, and you will need to define how each element is to be calculated and be able to define what is included and what is not included in each element.
Also, consider how and when the contract will be terminated. If you are unhappy with your contractor, you might want to be able to terminate the contract at your discretion. If you are prepared to take the risk a long term commitment might allow you to drive a harder bargain. Consider the risks and the benefits!
There are lots of standard contracts for maintenance work, but you need to decide how sophisticated you want it to to be, before you select a contract. Then get some professional legal advice on the best type of contract to use.
If you are getting your brother in law to cut the grass then something informal might be appropriate, but If you want to maintain a space station then get professional advice.