One can also be very extreme and get a Fiat two cylinder job. According to youtube reviews, it made nice sound and not too slow, hack it will be faster than the '70s 1.5l. This new 2 cylinder engine generates 105hp and 107 lb-ft at 2,000rpm, probably weights like 185lbs? It could potentially be over 100 lbs lighter than the 365lbs (engine+tranny). Fiat 500 weights around 865–980 kg (1,907–2,161 lb), with the twin-air, 0-100km/h in 11.4 sec.
So with Ford 3 banger's 125hp and 145 lb-ft, a 3 cylinder X1/9 probably will be an upper 7sec 0-60 car? Just slow enough to be pushed hard without getting into trouble. Like going from I-280 to Skyline Blvd. via Woodside village, then return on Page Mill road in LAH, or my favorite Hwy 74.