That sucks. I dug out (I mean literally dug, it was grown in under the plum tree really well!) my dads old 3.5hp push mower. Theres nothing special about it really, its just old, my dad bought it brand new, was used since about 85 or soemhting (ill get back to you on the date), but the points dont work, the plunger will not plunge!)
It seems weird, but I really wanna fix it up, theres next to no paint on the motor or deck, its rusty all over, but its kinda got sentimental value to me, even though my Father keeps telling me I might as well get rid of it. Its not like I really need it either. But I wanted to but these dethatching tines you can buy for push powers and put it to use for that! He alwyas thought it was 2 stroke, came as a suprise to him when I told him it wasnt. So its been well oiled too. I may just ditch the points and go electronic. Saves me alot of work!
Anyway, back to topic, have you can get another engine or parts, 3.5s are very common, shouldnt be hard to find one on the kerb or for cheap.