The ModExpInput creates integer representations of base, exp and mod. This is a mistake which prevents creating valid and cheap inputs.
E.g. you can create an input of base 0, mod 0 and non-zero exp of length 2**64 as:

0000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000010000000000000000 0000000000000000000000000000000000000000000000000000000000000000 80

Similarly, the Spec.calculate_gas_cost() is impractical because it takes the exponent : bytes parameter.