Personally i wouldnt be caught dead in a seafire against a 109G, its a mismatch in my book. However, i think you have two basic options (not necessarily mutually exclusive):
1. Sucker him into a turn fight (assumes 109 pilot allows you to do this). You should handily be able to out turn it. Keep it nice and slow the 109 doesnt have the nice low-speed handling the seafire has or the tight turn radius at slow speeds.
2. Tempt them into an overshoot and tag em as the extend out. AKA the "sucker punch". This is prob your best bet, and requires excellent timing and good anticipation skills. If the 109 wont turn fight you, he will be trying to sucker you into sticking your nose up in the air wherein they'll try clobber you. The trick is either sap more E from the 109 than they think they've lost and/or keep your speed/E higher than they think you are/have. Let them bounce you from the rear quarter (from a high angle-off-tail), and simply barrell-roll along their anticipated flight path, or rolling sissors, when they overshoot, and they will you will have enough E to tag em as they extend or follow them up their zoom to knock em off their perch, or even force them into a defensive. But you have to be a good judge of relative E stakes.
There is a slight variation of option 2 wherein you allow him to bounce you at very high speeds. All 109s suffer badly from sluggish controls at high speeds. You can use this to your advantage with the option 2 as above.
