Maybe you saw this already?
In the comment section, someone used
some man made fire brick for the lava rock.
Good to have some substitutions or a back up plan, as you probably already have.
http://www.alohafriendsluau.com/imu.html1) Dig a large hole but do not dig it too deep or it will make removing the cooked pig more difficult. For a 200lb pig the hole should be approximately 3' wide x 5' long x 1' deep.
2) Place newspaper and kindling on the bottom of the pit and then stack mesquite wood on top of that, extending at least 1' above the top of the pit. Mesquite briquettes can also be used. Arrange the lava rocks on top of the wood pile. The rocks should be of sufficient quantity to cover the bottom of the pit after the fire has burned down and also have enough to fill the stomach cavity of the pig. Set the fire and let it burn to coals. Use volcanic rocks otherwise the rocks could explode once heated. The fire should burn 1 ½ to 2 hours. The rocks should be HOT, preferably WHITE hot.
3) Set some hot rocks aside to put inside the pig. Rake any remaining coals to one side, leaving the hot rocks bare. Put down about a 6 inch thick layer of banana or ti leaves on the hot rocks, then a piece of clean chicken wire and some more banana or ti leaves, then place the cleaned gutted pig on top of the chicken wire and leaves. The chicken wire will make removing the tender cooked food a lot easier. Fill the inside of the pig with hot rocks. Some people like to line the inside of the pig with ti leaves prior to inserting the hot rocks. Add any other food (fish, shell fish, vegetables) you wish to roast on top of the chicken wire along with the pig.
4) Have a minimum of 100 loose ti leaves cleaned and ready for use. Put a single layer of ti leaves or taro leaves over the food then a thicker layer of banana or ti leaves. Well soaked burlap bags (50 minimum) can also be layered on top of the leaves and the entire mound is then covered by several layers of plastic sheeting as thick as you can find. Put a generous amount of dirt around the edge of the plastic so that no air can get into the imu and so that NO STEAM can escape from it. Shovel dirt over sacking and cook at least 8-10 hours or more. The pig should actually be put in the night before your celebration then dug up the early afternoon of your celebration. Check for steam leaks while the food cooks because if you loose heat then the food will take longer to cook. Dig up the food and enjoy! The imu takes 3-4 hours or more to create and is hard work. Be sure to allow enough preparation time.
FYI- Pork typically takes about 30 minutes per pound to roast in a 350 degree oven. The internal temperature should reach between 160 degrees for medium and 170 degrees for well done.