Is it not an option to just adapt the parts from your electric start 43cc 2-stroke to the Honda GXH 50?
As for Fuel tank options look to the world of mopeds. Moped Army - Swarm and Destroy
look thru the photo gallerys to see which models have tanks that you like and then look for them used on e-bay or at moped shops. The Deringer tank mentioned in the post above is from the General 5-star line of mopeds from the 70's and 80's. The Puch Magnum has a very interesting tank. There are NOS General Step-Thru Tanks availabe from handybikes.com Handy Bikes USA - The largest distributor or vintage moped parts in the USA!
for $50.00. And then as I said above ther is always e-bay they have a catagory for moped gas tanks.