I had a similar situation as you. I had a 2003 model boat with a smoking choking merc 115. Same issues as you fouling, loading up, etc... I repowered with a Yamaha 115 4 stroke. I am very, very, very pleased. I've trolled the thing for hours without missing a beat. Very easy on the gas. Very quiet. No pumping a bulb. Just look at the key and it starts.
Assuming you need a short shaft, For a couple hundred bucks you can get the new Yamaha SHO motor (super high output) which has a much improved holeshot and lower rpm performance. I purchased my long shaft version from Barling Boat Sales in Ft Smith. (Local dealer never returned my call). The standard (non-SHO) model doesn't have the low rpm pep of a 2 stroke. I'm not a run-n-gun fisherman anymore so it's not an issue for me.
My best friend is an ETEC guy and is pleased with his motor. He has a first generation model that had some issues but I think they have solved the early problems with that motor. Hope this helps.