Are there differences between the 2 and what are they if they are not interchangeable?
りき入ってんじゃん. For this sentence "(He) sure is putting power into it.", could chikara be used instead? E.g. ちから入ってんじゃん.
ちからが入る is a very common set phrase, which can mean both "(for a muscle) to physically strain (under pressure, etc)" and "to put much effort on something".
- (while practicing tennis) 肩の筋肉に力【ちから】が入ってしまっている。もっと力を抜いて、リラックスしてプレイしなさい。
リキが入る is a fairly uncommon and slangy expression. From what I understand, it usually means "to put much effort on something" or "to do something seriously". I may be wrong, and others may use it differently. I never use this phrase, but maybe so-called 体育会系 people tend to use this more often.
If there is no furigana, you can read it as ちから.