can i use a fg gas engine bottom end and fit my turbo ba head to it ? what sort of compression ratio will that give me .will this still work with the turbo .

it will work, but you will need to compare the cc of both heads to know if there is a difference..
the lpg engine runs about 11:1 and that's a combination of head chamber cc and piston. if the ba head has a larger cc chamber this will bring the comp down , but how much depends on chamber size difference.
otherwise with compression that high your answer is to run it on lpg or e85..

just something to think about is the fg runs a rear sump compared to ba/bf front sump, so you will need to use the ba sump and re locate the dip stick.. the problem ive heard is, early fg blocks had the provision for the dip stick front and rear and can be easily swapped over, but later fg only has it at the dipstick provision at rear of the block.

thanks mate i will have to see if they work because there is a complete lpg one for sale for $1300 with 15000 klm on the clock , would the ba sump fit and the ba oil pick up ? the compression is the killer

Yes the ba sump and pick up will fit the fg motor.. but like I said, later fg dont have provision for the ba dip stick

not a good idea

I have done this conversion. in short it will work, however it will ping its head off due to the comp. best way to fix it is to use E85 and get it tuned by a good tuner