Better the devil you know...
My minimatic when rescued from it's garden ornament status had only reverse and neutral..
To fix it cost me about $50, 12 years ago, all it took was 2 fwd clutch plates and a gasket. Easy fix, motor stayed attached to box.
`Reverse only' problems could either be the top and reverse clutch, or the reverse band may be worn and need adjustment, or there could be fluid leakage from the reverse servo piston..
But don't be scared of these boxes, they are not rocket science. A few people here on ausmini have totally stripped and rebuilt them at home.
See Todd Miller's site http://www.austinamericausa.com/ and look under technical advice for lots of info. Minimatic and Austin America/Morris 1300 automatic boxes are the same thing.
