I found a solution.
Adding a reference to VisualBasic gives access to Interactive.AppActivate, which can activate any process given a processid, which I already had implemented.
This page gave me the solution AppActivate In C#
I check for running process in the following manner
Process currentProcess = Process.GetCurrentProcess();
var runningProcess = (from process in Process.GetProcesses()
where
process.Id != currentProcess.Id &&
process.ProcessName.Equals(
currentProcess.ProcessName,
StringComparison.Ordinal)
select process).FirstOrDefault();
if (runningProcess != null)
{
Microsoft.VisualBasic.Interaction.AppActivate(runningProcess.Id);
return;
}