I have a Briggs 3.5 Vertical shaft engine on 70's model Hahn tiller. Motor gave out, trying to find build kit, no luck. Question, Can I take the Heavy flywheel off this old Briggs (1983?) and put it on a newer Briggs mower engine? I am aware of the balance issue that mower engines use the blade as a counter balance for the lighter flywheel. But hoping I can find out if a flywheel swap is possible..thanks for any info

Un-less it's identical, I'd suspect they'd be issues...  
 
If the other engine needs a flywheel, look up the part #, then do a search on Flea Bay, etc.  
Old Briggs stuff isn't terribly hard to find...  
 
What's wrong with this flywheel?
 
 
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