Conventional wisdom and a general acceptance of the premise that rear tires mounted on the front work very well on trike conversions.
If you check various tire threads (whew) you will understand that since your trike won't lean like the 2 wheeler did, your front tire will wear very quickly. Using a rear tire on the front because of it's different tread profile and deeper tread gives you the mileage advantage.
Since it is a trike, tread squirm is not detrimental to the rider(s) so a deeper tread that is not too aggressive is the best bang for the buck (unless you can find a car tire to fit).
Your size is very limited (100x90x19) for rear use. I did find an AVON Roadrider AM26 has a good looking tread compatible for front use and not too expensive. Hope this helps. Most bike shops can get this tire for you.