The car looks good in the photos and worth fixing. dino245 on this forum has a built up M42 for sale. Your profile doesn't indicate where you're from, but a good used M42 with 100K miles would be $1000 or less. People talk about S52's for $2-3K but the complete swap can go $5K easy if you do it yourself. There's exhaust, electronics, A/C, brake booster, and 14 other things to mod or upgrade you don't think of at first. It's easy for it to become the never ending story or a tragedy. I'd keep it original and spend a grand on it. It would be worth $5K easy if nice, and if you want more power, I'd talk to dino.