How many amps transformer do I need to charge a 75AH battery without overheating the transformer and rectifier.

Depends on how fast you want to charge the battery. 6-10 amps is plenty. Why not just buy a small battery charger? $30 at Harbor Freight.

Even a 2A hobby type charger would be plenty. It's not just the charging, it's also the over-charging and cell balancing that you should be concerned about--especially with Lipo batteries.

Buy a battery tender... it won't overcharge like a regular charger will, or even a float charger eventually will.