Are those inches or centimeters?  Coroplast is light but a model of 80" span w/16" cord won't weigh 21 oz.  I think you have to add 100 or more ounces to your estimated weight.   

I fly two models of that size - one on a 26cc and the other a 34cc (1.20 to 1.60 equivalent engines).  A .45 would hardly improve the free-flight glide.  They weigh 192 oz and 232 oz dry!