If you have a 2nd set, use your nice ones inside and your other set for the field. Marching wear and tear is really rough on your drums, and getting/finding someone to do timpani maintenance is hard/$/time consuming.
We had an indoor set that were really nice and a field set that were pretty okay and could be used as a second set inside if we hosted an event but primarily lived on the Timpani field cart for Marching and Indoor season.
I use the timpani sounds on a Yamaha DTX with an amp. The sounds are pretty realistic, it saves wear on the timpani, and it's a LOT easier than lugging four timps around to football games and contests! Also, it's reasonably priced and you can use it for all kinds of sound effects.