The F3 is a nice bike. You've got a couple of options -- you'll have to decide which you like best. The amount of work involved varies from simple to involved.
The easiest is the Targa Solo Seat Cowl from Targa (
http://www.targa-acc.com). Cost varies from $70 unpainted to $130 painted to match your bike.
Next step is a 'solo tail'. If you're running on the street you'll need a tail light and real race plastic doesn't have tail lights. Often the maker will have a street model that has light cutouts. Targa has tails like this. Generally, this model will fit over your stock seat and simply cover up the rear portion of the seat. I use this model (with race a race body) when I take my F2 to the track. Be aware that these can and do rub the side of the stock seat so unless you're really lucky you'll have a hole in pretty short order. I put black duct tape on mine to protect against the rub, but then I'm not really concerned with how pretty it is. (I also bought an extra seat for cheap on eBay that already had a tear in the back that is covered up by the tail.)
The F3 has a 'solid' tail, so as far as I know, anything beyond that would involve welding, etc. The seat has a big plastic 'pan' for a base. I imagine that you could fabricate something if you were really handy that would look like an F4i tail, but that strikes me as $$$.
Good luck!