What is the proper way to create an HTTP Monitor and set the alias service port to 80 via iControl API?

With REST API you can use that :

{
       "name": "my_http_forced_alias_80",
       "adaptive": "disabled",
       "adaptiveDivergenceType": "relative",
       "adaptiveDivergenceValue": 25,
       "adaptiveLimit": 200,
       "adaptiveSamplingTimespan": 300,
       "defaultsFrom": "/Common/http",
       "destination": "*:80",
       "interval": 5,
       "ipDscp": 0,
       "manualResume": "disabled",
       "reverse": "disabled",
       "send": "GET /\r\n",
       "timeUntilUp": 0,
       "timeout": 16,
       "transparent": "disabled",
       "upInterval": 0
    }