Ripmax Firebird XL Battery ?

Just picked up a Firebird XL to get back into flying after 20 years, The original battery here is a 7.2v Ni-MH 900mAh which is long past its lifespan! 
I'm looking for an equivalent modern battery ( Lipo ? ) of the same dimensions, that will fit inside the model.  

The size is roughly 5 x 3 x 3.5cm . 
Can anyone recommend a suitable battery ? also, I have no experience with Lipo batteries or chargers, I only have old Nicad and Ni MH chargers here ! 

cheers !

If you go to a LiPo the problem you will have is it will be too small and light to be a 1:1 replacement.
A typical 1000 mAh 2s (7.4V nominal) is 68 x 30 x 14 mm and at 54 g is about 1/2 the weight of your 900 mAh NiMh.

You could always add some lead to make up the difference but much better would be to increase the capacity of the 2s LiPo so it equalled the weight of the NiMh. It might need some ingenuity to get it into the available space but it would likely double the flight time on a charge.

I suspect your Firebird has a brushed '380' motor. Rather heavy for what it does. A brushless motor of the same power is about 1/2 the weight and a bit more efficient as well!

Given the age of the Firbird XL design there are later designs that are more practical to fly and easier to get spare parts for..
On a web site a poster praised the Firebird XL as a beginners plane but pointed out it was no longer in production and that was in 2003! :woohoo:

Also - if you go for a lipo, you will have to buy a different charger as well. A nicad/nimh charger will make it go bang!
It's not just the weight that will be different, the balance point will have moved too, and may make the model unflyable anyway.
Does the radio gear still work?
You might find it cheaper to hang that model on the wall as a decoration, and start new with modern kit and models.
Have a look at the Bixler, or the various rip-off copies of it. Easy to fly, and a pusher motor, so you will find it hard to break the prop!

If you really want to get your Firebird XL flying "just because" you can get a new 900 mAh 7.2V NiMh pack. They are/were used by the RC vehicle boys but are now little used so are relatively expensive compared to a similar sized LiPo.
This one is on Ebay

Also be aware that if you opt for the Lipo battery type you will need a suitable "speed"
controller as without one you are very likely to damage the battery without it.

FTB
Surely you only need a new speed controller if you also change from brushed to brushless motor. A 2s LiPo voltage is very close to that of 7.2V NiMh.

It depends on what the motor shut-off is set at, if it has one. It might just run until the battery is exhausted, and that will kill a lipo.

Eric gave the answer