A scale of 1"=100' would be a 1:1200 scale plan--let me ask, exactly what is this a map of?
The ratio of 1:1200 to 1:87 is about 13.8, so you could multiply the lengths of things you wanted measured by 13.8 to get their size in HO, or you could use an engineering ruler to measure things (a ruler showing tenths of an inch would measure things in increments of 10 feet) and then once you know the "real" size of an item you can draw it out with an HO scale ruler.