Serial to Intouch|OPC UA Standard|Forum|OPC Foundation

I have a problem and looking for help on how to go about it. I am a system integrator and working on a job for a city. They have very old equipment and not much money to replace anything. The equipment I am talking about is a ScadaPack, just ScadaPack nothing to added to it like 32 or 357, it only has serial modbus. The job I am doing for them is replacing a old computer with a new one. The Problem is that the new version of Aveva Wonderware Intouch doesn't support serial modbus anymore. They recommend me to use top server, but it charges 1500 a year. My company is small and can't cover that. 

So the main question I want to ask is doesn't anyone know of a cheap program that can take serial modbus and convert it to something the new Intouch can read. I think that TCP/IP could work or maybe an OPC again I think could work. Please keep in mind I am just a controls engineer and only know just a bit about computers. I know more about PLCs and the like.

Please help!!!