I make a batch of venison haggis every year. I use the haggis kit from Weschenfelder. Comes with Ox bungs and premixed seasoned oatmeal. Works every time and is as good if not better than any haggis I have bought. I don't like to waste anything from a carcass!
Haggis can only be made with mutton offal! What you're making is an abomination, ye Sassenach
In truth though, the only time I tried using venison offal I wasn't impressed. Mind you, that's probably because I make a damn good haggis myself & the venison version was just too different from what suited my own blend of spices