I've been trying to find a 21" front wheel with 25mm bearings for my 09 softail but all that seem to be available are ones with 3/4 bearings. Can I replace the 3/4 with 25mm ones and is it a difficult conversion

Mate, easily changed, there are conversion bearings available....though I will mention that some Harley wheel bearings are propreitary and may only be available thru specialist suppliers such as All Balls in the US rather than regular bearing supply shops. Have a quick look on eBay (US) and you will find what you are looking for.