The Sāṃkhyakārikā is centuries later than Early Buddhism, approx. half a millenium or more. It is from the early Christian era.
It seeks to posit itself as a text in the lineage of early-Sāṃkhya tradition, of which no prior text has been so far discovered, so one is unable to validate to what extent the Sāṃkhyakārikā is representative of early-Sāṃkhya. It appears that early-Sāṃkhya became quickly the pre-eminent philosophy of late-Vedic India, so much so that it was seen as a kind of template for all later darśanas (philosophies), and its founder Kapila becomes so venerable that the god Kṛṣna, an avatar of Viṣṇu in the Mahābhārata (Bhagavadgīta 10.26) identifies himself with Kapila, the foremost of munis (sages) - “siddhānāṁ kapilo muniḥ”. So Kapila is described as a siddha and as a muni and as a buddha.
Based on this background knowledge, I have speculated earlier here, here, here etc that perhaps Early-Buddhism itself was Early-Sāṅkhya and the historical Buddha - who is also known as a Buddha, a Siddha (siddha-artha = siddhārtha), a muni, etc was the same person who is identified in other Hindu philosophies as Kapila. The name Sāṃkhya (derived from the word saṃkhya = counting) means a philosophy that enumerates - which is a name that very closely fits Early-Buddhism – considering the phenomenal amount of enumeration that happens in early-buddhist philosophy - such as the noble-eightfold path, four-noble truths, 12 links to dependent origination, five aggregates, the three marks of existence, the five hindrances etc.
It would have not looked like anything at all. Practically everything in Early-Buddhism is late-Vedic in geography, time and culture - that to ask of what pre-Vedic buddhism would have looked like would be like asking what would a child have looked like in its pre-parental stage.