I think you should give it a go. I have never restored a Sei but have done quite a few other Italian bikes. Start using penetrating fluid on the plugs and try to move them each day- it may be that you will have to wait for engine removal to get at the middle plugs properly. Also get some fluid into the bores via the ports and try to kick the engine over each day as well. After you get the plugs out and the engine free I would get the parts you think you will need. The pipes will be expensive but seeing that the bike was given to you that should be OK. I generally prefer to try to go back to original but that is a personal choice. Another personal choice is to use original fasteners and not stainless. I try to do a bit every day no matter how small. Take plenty of photos before you really start and also as you go. The more the better. If you post a picture it would give us an idea what you are up against.