I think my Alternators gone !!!!

[Ollie]

Hi folks, 2 days ago i went for a spin in my MGZS 1.8 120 and parked up in the supermarket car park. On returning the car would only make a rapid ''clicking'' noise when i tried to start it. The breakdown man arrived and applied the power pack - the car started. Checking across the poles he said he thought it was just a ''naffed'' battery i.e. not holding charge. I managed to get home ( 4.5 miles ).
I purchased a new battery, fitted it and the car started up straight away.
I thought i'd take it for a drive but after around 6 miles the battery and engine warning lights came on, so i thought i'd head for home. 2 miles further and all power was gone and the car stopped. 20 minutes later i tried the ignition and to my surprise it started, so off i went. On the way home the warning lights were still on but started getting dimmer just before i got home.
I believe the alternator is at fault and so have enquired and managed to get a quote of £40 from a scrap yard for a replacement.
The problem is the replacement is off of a MGZR. Will it fit? and will it be adequate??
I've posted a few questions in the past and know you're a great bunch of folks, i therefore look forward to your help.

Cheers,
Ollie.