My reccomendation is build a Jake Raby Type 4. You can use the engine you have (crankcase anyway), and you won't have to mess with the body, transmission, or anything else. You can easily get the 150 HP you are after, and better yet, loads of torque. That is the route I went with my 1975. My 1.8 liter is now a 2270. Be prepared to spend ~ 6K on the motor ,with you doing the work.
The best thing about this decision is the car will still look and mostly be original, and will still be the light, air cooled, nimble sports car it was designed to be, only a hellova lot more fun, when your back is pressed back into the seat by all the torque. The only thing I miss is fuel injection, which can also be added with some more $$. The sound of a 6 is also nice, but the 4 with 4 weber throats sitting about 18" from your head
sounds pretty good too.
Good luck with the decision.
Keep us posted.
Rob in Overland Park, KS