27000 Buck Firebox Advice

I bought a 2400 square foot cedar house last year with a 27000 Regular Buck free standing stove in the basement. The house was built in 1976, and I assume the wood burner is about as old. I have been using it 24/7 this winter to supplement the oil furnace with a lot of success. I recently noticed cracks in the fire brick. After investigating, it started crumbling. What do you recommend I replace this with? The shell seems to be in great shape. Thanks...

Most wood stove retailers will have firebricks for sale, they are nothing special and usually all the same size.

Thanks for the response... I've also been advised that it might be better to pour a Portland base than use firebricks. Any thoughts on this? Have a good recipe for the mix? Thanks for your help...

I have a 28000 buck. It only has blocks on the bottom. Is yours on the bottom? You can always call the link below. They are very informative for the old bucks
http://www.servicesales.com/universal-wood-stove-parts-c-28.html

Thanks for the reply... It had two 1.25 inch pieces that covered the bottom and 4 inch high block covering both sides and the back. I'm just looking for the best solution to keep this old but good burner going. Thanks again... When I talked with the people from Buck, they pushed me towards the Portland products...