Set custom display form for SharePoint list
Set your custom application page url on content type and if your list uses this content type, form will be associated too. If trying to set this application page url to list property DefaultDisplayFormUrl, I got errors of SPForm file not found
Add-PSSnapin microsoft.sharepoint.powershell -ea 0
$webUrl = "https://yoursitecollectionurl"
$ctName = "Item"
$web = Get-SPWeb $webUrl
$contentType = $web.ContentTypes[$ctName]
$contentType.DisplayFormUrl = "_layouts/yourLocation/YourCustomApplicationPage.aspx"
$contentType.Update($true);
$web.Update()