The money you have spent on this car taking it backwards sounds like a mistake to me, I would do one of two things
Sell it and move on, you have a car that seems to have been through some poor workshops in its time and the love seems to be lost from reading your posts, and I know from bitter experience that the car will never be the same once this happens
Bit the bullet, get it to a good gargage to access the car fully and get it sorted, It seems to me that if you get the single working you have a good base car, I would fit a second hand 5 speed and run modest boost levels, it should be ok, I fear the fitment may have been the issue in your past experience, I have to add that getting it sorted at one of the better tuners will not be cheap so think abut the above option before you decide
Can you tell us what is wrong with the car right now, apart from having no gearbox ?, i might have someone who would be interested