The rifle is a good backyard plinker, but not really suitable for the quarry you suggest, except for rats at close range (10-15 yards).
Probably best to get some more money together and buy a more powerful and accurate rifle. Not worth refurbishing unless as a plinker, if you decide to sell you will not get back the money you spend on parts so best to sell and get a different rifle. These guns go for £30-60 in used condition.
You can get parts for this Milbro Diana from Chambers.
http://www.gunspares.co.uk/shopdispl...=24470&cat=G79