Re: My Felt 1903 build progress
You will have to vent the gas tank in some manner. My cap looks about the same as yours. I used a brass fitting and a hose like on a dirt bike. It seems to work well.
I also experienced some flex in the bottom bracket under load. I reworked the motormounts to stiffen the area and it seems to have solved the problem. If your bottom bracket flexes you will feel it in the peddles so keep that in mind when you first try it out.
I look forward to seeing your progress!